First of all,why you want to hack a webpage?Is it a certain webpage or any site at all? There are many reasons to hack a website, or a webmaster.Maybe you want to take a revenge or maybe you want to have fun or just learn how to do it ! You can deface the website which means replace the original index with a new one or you can gain access to the member area of the site which might be easier.
Brute forceTo do that you need a brute forcer or brute force attacker and some word lists,the brute forcer sends multiple user/pass requests of words that picks up from namelists and tries to hack the account untill it does! So lets say imagine a porn site that asks for a password , you go there you copy their address , you add the address in a program called brute forcer and then from the brute forcer you choose a text file with names to be used as usernames and a text with names to be used as passwords,the brute forcer will try untill it finds a correct user/pass This should be easier for the newbies than exploiting cgi bugs , many of the newbies havent even heard of it i hope i didnt confuse you with this tutorial there might be more tuts about web hacking and cgi bugs and such.Till then try to find the way to cgi bugs yourself with the cgi scanners in the Web Hacks section or download a brute forcer to crack accounts.