I take it from the questions this is the first time you`ll be starting a website?
Ok you have a theme or niche so thats your first step. Before you buy anything the next step it to have it clear on what you plan is once you have it up and running, write down!
A sort of business plan to your self, it will be easy to do if you just get out a sheet of legal paid and jot down ideas, everything from the name, the main idea behind the forum and goals. If you plan to make money from it decide how, via memberships, ppc ads, banner ads ect.
Then once you have that down organize it. I know this sounds silly but remember if you fail to plan you plan to fail.
Do you want to make money from this site? If so you should go with a pay forum script, and if this is you first time get one that has tech support and will help you install it. The paid scripts will have a lot more features and be much easier to add advertising. At least thats my opinion on most other scripts I`ve seen.
If its just a forum most PHP scripts can be customized from the admin panel. So you wont need to build a site.
As far as links to see the scripts you could try www.turn-k.net they have a script called k-rate that could work for you. They also have different versions starting from $59 to $129 depending on the features more importantly they have a demo you can install on your web server to try first.
Other than that go to hotscripts.com or just do a google search for twitter clone or fmylife clone.
You`ll need hosting also and the domain. If this is your first go at it try a free domain like .co.cc or .cz.cc also so it would be www.yourname.co.cc and look for a free hosting service that dose not require to to host ads. Again google will point you in the right direction. Later if you start to make money and want to get a .com name you can forward the free .co.cc to the new so you don´t lose traffic.
Also for a few bucks a month look into hostgator.com
Hope this help