[nmader]

Well, you can't exactly edit a script without getting to know the script. So your best bet to get started is to grab a notebook and right down features you would want to change and how you would want to change them. Then you can start to edit them and refer to the SA:MP forums (and especially the SA:MP Wiki). Maintaining a server is rather simple: if you have money, pay for a host, if you don't portforward and keep your PC up 100% of the time. Fix bugs/errors by referring to the SA:MP forum/wiki.