One of the most important parts of search engine optimization is keyword research. If you don't know how to do this you are going to have a real problem getting your site to rank well. You will need to decide which keywords you want to rank for, this can be quite a complicated task so it is important to learn how to do it.
In most cases the first step to keyword research is determining how many searches there are for the phrase that you want to target. This is the easy part as almost any keyword tool will tell you how many searches there are each month for the phrase. The problem is that the numbers are an estimate and can vary wildly from one keyword tool to another. That can make it hard to estimate how many people actually search for the phrase each month. The best idea is to look at how many searches there are for the term relative to the other terms. The numbers themselves may not be accurate but the are usually a pretty good guide to which terms are most searched for. There is a great deal of debate as to whether you should target terms that get lots of searches but have lots of competition or terms that get only a small number of searches but have little competition. That is something that you will have to decide for yourself.
The next step is going to be assessing how much competition there is for the keyword that you want to target. The traditional means of doing this was to determine how many other sites are listed in the search results for that term. This is not really a good way to determine how strong the competition is. The problem is that there is little value to being ranked if you aren't on the first page. Therefore the only real competition is the sites on the first page. That means that you need to find out how tough those sites are. One way to do this is to look at the Google page rank of the sites on the first page. Basically if most of the sites have page ranks of three or less you can call it uncompetitive. While page rank isn't actually used for ranking sites it is a good proxy for how strong Google thinks the site is.
In most cases the reason that you are trying to get a site to rank well is to make money. That means that you are going to need to have a means of determining if a phrase will be profitable when you are doing your keyword research. In a lot of cases this will just come down to common sense, phrases about specific products will usually be profitable. Buying phrases like who sells the cheapest widgets will usually be profitable. One good way is to search for the term with Google and see how many ads come up with the search results. If there are lots of ads the term is probably profitable or there wouldn't be people paying for all of those ads.