Once you decide on your keywords, it's very hard to make changes later on as you already made the investment. The first thing you want to do is keyword research. Once you identified your niche and the most commonly used terminology to describe your services, you need to compare the keywords in all combinations to see which ones produce the most traffic.

You have to separate 2,3 and 4-word keywords as it is easier to rank for longer tail keywords. If for example, a 3-word keyword produces similar traffic then the best 2-word keyword, then you want to rank for that 3-word keyword. The only way to be sure is to do rigorous SEO and keyword research.