First, you should define what is the main subject of your Q&A website. Is it Q&A about job interviews, games, movies, or a general one.
After defining that to yourself, you can search for similar websites, and start your research.
Firstly, research what are the topics they are writing about the most. As well, search their domain in websites like alexa, where you can see the top keywords which lead to their website from search engines.
Secondly, try and search the content they are uploading in google - which rank do they get? If it's high enough, try to figure out why, and how to implement it as well - maybe they have a specific way to write their titles, which tags are they choosing etc.
Now that you know which content to write about in your website (as we know you must generate content in order to people to come), you need to keep them in. In order to do that, I learned it's very important to keep your website clean. Do not fill it up with advertisements, many useless plugins etc. Keep it clean, and to the point. People who will come back, are the ones who got what they were looking for in your website, with no extra hassle from your end.
One more thing you can do, is improve the style of the website - make it comfortable, and easy to work with. Keep it clean and with no extra attractive colors (again, you can look at competitor websites in order to get some inspiration).