Your Q&A pages will automatically be optimised for SEO (users are asking questions that other users will usually be typing into search engines anyway). If your looking for broader generic terms related to your Q&A site you'll need to do keyword research and have optimised landing pages.
Optimising landing pages means writing for users first and foremost but then weaving in your primary keyword once in the meta title (usually at the start) and then once in the standfirst (first paragraph). You can weave in synonyms as well. But your copy (content) should have this anyway.
Keyword research is a skill and needs to take into account competition and your propensity to rank. I'd start here
https://adwords.google.co.uk/KeywordPlanner or
http://www.bing.com/toolbox/keywords (or paid
http://www.semrush.com/ is quite good) and then if keywords seem too competitive look for longer tail versions of them here
http://ubersuggest.org/
I've broadly summarised what are complex areas, so do your homework (research) fully first if you want to see results and always play the long game in SEO (expect your work to take many months to come to fruition).
