When a person makes a search they use certain keywords and phrases to find what they want. In case your content doesn't include those keywords or their close variations, it most definitely won't every show up in that search results page. You can write your content without doing any research and hope for the best. This approach works when you don't care that much if your content is found through search engines or not. When you really want to rock your search engine optimisation, use keyword research and planning to find out which keywords are being searched and target them accordingly.
You can target short tail keywords, which are very wide, common and very high volume keywords. And you can target long tail keywords, which are more narrowed and specialised, lower volume keywords. Short tail keywords are usually harder to target. While they have a higher search volume, there are also many more websites that use those keywords. Long tail keywords may be easier to target, because there might not be as much competition in the ranking with them. Long term keywords are, in that sense, much better for smaller and newer sites. That doesn't mean you shouldn't ever try to target short tail keywords.
Whichever way you go, making keyword research helps. You don't have to try guessing which keywords are being searched and what sort of search volume they have. Even though SEO is mostly trial and error, and there's no magic bullet guaranteed way to do it, doing keyword research can be used to improve it.
Keep in mind, your search results are not the same as they might be for another person. Search results are optimised based on your language preferences, location, your previous searches, the websites you have visited before, the device you are using to make the search and many other factors. You can test this by making a search with the same keyword using several different devices and browsers or asking your friend to do the same search as you do and share the search results they get. You will soon notice that the search results differ greatly by the content that's being offered and by the order of the results.
Free keyword planning tools
There are several ways you can go about your keyword research. First you can try just doing a search with the keyword or keywords you are targeting. See what sort of websites and articles you are seeing in the search results. Again, these are the results you get, based on the many different factors, but they give you an idea of the content the search reveals. I also recommend checking out the related searches, which are located in the end of the search page on Google and on the side on Bing. The related searches can give you further ideas of the keywords and phrases to use in your content.
For example, I just search with Google "keyword tool" and in the related searches I found such phrases as "how to use google keyword planner" and "keyword generator". While "keyword tool" has a higher search volume than these two other phrases, I might be interested in targeting them if I want to get the pages and articles of my website to rank better in some searches.
The search engines often provide their own keyword planning tools. Google AdWords Keyword Planner requires you to have an AdWords account, which you can set up very easily. You don't have to pay for it, but you'll have to give your billing information to set it up. Using Keyword Planner may seem complicated at first, but it really is very easy. With the Keyword Planner you can find new keywords based on your product or service, your landing page (they page you want to optimise), or your product category. To refine your keyword research, Keyword Planner allows you to target the keywords to a selected language and location. Additionally you can do a research in keyword search volume and trends, or use keyword lists to find new keywords.
To get started with your keyword research, choose "Search for new keywords using a phrase, website or category" and add your keyword or phrase in the first field that says "Your product or service". After this just click the button that says "Get ideas" and you are given a list of keyword suggestions. The keyword suggestions include information about the amount of monthly searches done using the specific keyword, if the competition in advertising with the keyword is high, medium or low, and how much you would have to pay for an ad to be shown with they keyword. Keyword Planner is intended for researching information for creating Google AdWords ads, but it also gives useful information about the keywords in general.
Google also has a tool called Trends, which can be used to find out how different keywords and topics are trending in searches, in which countries they are trending, related topics and search queries. You can choose of which time period you want to see the data. By default it appears to be showing 5 years, but you can use one of the presets (past hour is the smallest timeframe) or a custom time range.
Bing Ads Intelligence requires Microsoft Visual Studio Tools for Office and Microsoft Office Excel 2007 or newer installed on your computer. It is a free tool that is installed to your computer and it is used to create Excel tables with keyword data. I haven't tried it, because I don't have Microsoft Office on my Mac.
Keywords Everywhere is a free browser add-on. You can install it for Chrome and Firefox and it utilises other keyword research tools to give you the keyword data straight on the search results page. After installing the tool, you will need a free API key which is sent to your email address. As you have validated the API key, you can do a search with Google and it gives the search volume and other data right there on the search page.
Premium keyword tools with free versions
Keyword planning and SEO are such a hot topics, that there are many paid tools out there for them. For example, webtexttool seems like a very interesting SEO tool. It allows you to write your content and gives then suggestions on how to improve it for SEO. The tool is paid, but offers a free, 30-day trial. Unfortunately the trial requires giving payment information.
Link-Assistant.com is a premium SEO tool, which includes keyword research and ranking tools. They offer a free download, which they say is without time limitations and doesn't require credit card information. Installing the tool on a Mac requires installing Java.
SEO consulting company Moz offers a 2 free keyword queries per day with their Keyword Explorer. Keyword Explorer, amongst other things, gives you information about the difficulty of ranking higher than the current competition on the first page of search results. Higher the difficulty is, the more difficult it is for you to get on the first page of search results for this particular search keyword or phrase. You can also get a free 30-day trial for Moz Pro, which also requires giving credit card information. If you are fine with that, why not.
With Serpstat you can make 30 queries per day for free. The tool gives pretty detailed information about the keywords. You can also use the tool to analyse the keywords of your site and their search position and volume. With the paid versions you can unlock more queries and get access to other features and more detailed analysis.
Wordtracker offers suggestions for long tail keywords. It has a limited free version and with paid version you can unlock more keyword suggestions and data. It gives information about the search volumes, count of websites where the suggested keyword appears in the title and the backlink text, competition and keywords "effectiveness".
For limited Google, YouTube, Bing, Amazon and App Store keyword data, try Keyword Tool. It gives suggestions for unique keywords, but to get the data, such as search volume etc., you will need to get the paid Pro subscription.