If you go to Search Console under the Acquisition tab and then click Queries, you’ll be able to see different searches (aka keywords) that users found you through. You may have to set this up, but it’s easy to do and only takes a few minutes; you can see how here. Use this information to evaluate and monitor your keywords to see what’s working and what needs to be changed.
In order to know which keywords to target, it's essential to not only understand the demand for a given term or phrase, but also the work required to achieve high rankings. If big brands take the top 10 results and you're just starting out on the web, the uphill battle for rankings can take years of effort. This is why it's essential to understand keyword difficulty.
In a nutshell, Long Tail Pro helps you quickly find keywords in bulk based on a seed keyword that you input.  In addition to returning hundreds of related keywords, the tool also shows search volume (monthly search volume), advertiser bid, number of words, rank value, and my favorite Keyword Competitiveness (helps you judge the competition and keyword difficulty).