depends on where you live, you can even pay by SMS if you live in USA I think.
I suggest you to visit
http://www.runescape.com, click on "Subscribe" and you will have to log in to your secure meny, just type in your name and pass, then you can read all about membership
That would be the easiest and best way to answere your question. If people try to explain its a big chance that someone says wrong or misstakes or something like that, but if you visit the page you get a simple but good answere on your question.
~Hello5000