Preface: There is NO guarantee that your site will be successful.
In the following you find answers and ideas, I also point to some of my premium plugins:
1. Check the most popular q2a sites to get an idea of what they are about: http://www.question2answer.org/sites.php (use google translator if you don't understand the languages).
2. Create a community, gather people with knowledge and passion (this task is tough).
2a. Play the who-is-the-best-game. E.g. use the best users plugin (pro).
2b. Reward them. Example: I am giving out 6 awards per month on http://www.klaustukai.lt/ and 4 awards on http://www.gute-mathe-fragen.de/
2c. Be a social person, connect and engage.
3. Have a fast website (ranks higher in Google). Use advanced themes that are responsive for mobile devices, e.g. q2apro-theme diligent.
4. WRITE great content that people need and share. Experience: I paid a ghostwriter for 10 months. The max. audience I can ever engange is 4 mio. people - which are speaking Lithuanian, quite small, but I managed to Alexa Rank ~1 Mio. and local ~4000.
5. EASIER: Focus on one topic, not hundreds. My specific math forum got one of the biggest in Germany (currently 32786 questions), the general forum in Lithuanian must still grow (about 3500 questions).
Opposite to others, I do not recommend the use of social networks. Check out *the* biggest forum ware: stackoverflow.com and stackexchange.com - they do not use FB or G+.
And again: Focus on great content, this excites people and make them come back!
Hope that helps :)
PS: Enhance your users' experience by installing free and premium plugins, see http://www.q2apro.com/plugins/ and http://www.question2answer.org/addons.php