First thing is to make sure you code is optimised with the necessary META TAGS.
Then submit the site to as many search engines as you can. It could take a couple of months until the site gets listed, but it will. Just don't keep submitting all the time to the same search engines as this can lead to them not listing the site.
Do a search on different search engines with the most important keywords that you use for your site. Visit these sites that are returned and if they look good ask for a reciprocal link. Basically you will link to their site if they will link to yours. Get plenty of these reciprocal links with other top ranked sites and it will do wonders for your ratings.
Make sure your sites content is regularly updated. Give people a reason to keep returning over and over to your site. A forum is a good idea if the subject of the site is something that people will want to talk about. It's also a great way to find out what they want from your site and so you can constantly build it to their needs.
As more and more people visit and stay, they will start to advertise the site for you. Look how many links to other sites you see in these forums.
It takes a lot of work to start with, but does get easier with time. As long as you don't rest on your laurels and keep it regularly updated it will go from strength to strength.