Try Google keyword research tool, Google Trends and Google Insight. Moreover, don't forget to analyze the competitors website.
I find the best way to do it is to think up a few keywords that best fit. Then I google them. The top 3 web sites that show up? I look at their metas and see what they are using. From there I can pretty much discern what combination of keywords may work best. I try to base it on popularity, relevance and frequency. I try to use no more than 3 main keywords in my alt tags. Generally they are variations of, or non-plural combinations of the keyword. Doing this helps grab a piece of the less used market, since the most common keywords tend to be difficult to rank for. YMMV
First think for a moment what should you write to search your services or product? Make list and then compare that list to Google adword tool. Also check your competitors keywords they really helps you in selecting yours.
use this site for keyword selection https://adwords.google.com/o/Target...000&ideaRequestType=KEYWORD_IDEAS#search.none