Posted in this forum is a sticky-thread linking to the
Useful Links you requested.
PhrozenKeep is an excellent source of D2 modding information, although this forum does have some commonly answered questions if you read around.
For me, I got started by getting a program to unpack .mpq files (these are just a variety of compressed file, like .zip or .7z), then a spreadsheet program (I just use OpenOffice Calc) so I could edit the text files in the /data/global/excel folder, and then reading the
file guides and files to puzzle out how things worked (similar to a relational database). To see your changes start D2 using the "-txt -direct" flags.
If those steps sound too technical for you right now (expect about a week of figuring things out before you can actually see the changes you're making), then consider just preparing a list of things you'd like to change so that you don't forget something while learning how to modify stuff (it took me about a year to implement the things on my list, but I wasn't too hurried).