Advertising comes first. Then maybe when you have 300 - 1000+ users then you can start competitions and giveaways. Don't promote your forum by saying that people will get a prize because in todays world, people will do anything just for a prize and then forget about what they signed up for to get it and just focus on the prize. Remember. Advertsing. First.
Giveaways are okay as a temporary measure but keep your focus with Social Media Marketing. Use Facebook, Reddit, etc to promote it. Also you can use services of good SEO as suggested by one of the members above.
