Script File Permissions

I've been trying and trying to get my site to work lately and all but one part does. Of course that one part is the most important part (it always is). My problem is probably rather simple to most of you, but I don't know how to get around it: My script does not have permissions to write/append files or just flat out create the files; it can read any files (accessed "files//nameoffile.xml"). Originally I was working with text files, but that didn't work either (and had the exact same problems). So my question is this: how do I get my script to have permissions to write or append to any file?