There already tons on the forum. It all depends on how you want to achieve it, on how the website is vulnerable or on how much your hacking skills are advanced.
Some vulnerabilities that might provide server access:
- XSS;
- CSRF;
- SQLi.
You might get access by also bruteforcing, or dictionary attacking the access panel. You can get access to the access panel with a few tools (our friend The Alchemist coded one for the forum too, just look for "The Alchemist Admin Finder" in
http://hackcommunity.com/search.php , or you might also Google Admin Finder by Reiluke).
Also, by directly knowing the server admin, you might get access with stealers, backdoors, keyloggers.
This is just a brief introduction for you; by giving us more we might give you back too.