There's no real way to learn RGSS2. All you need is dedication. I find it easier to modify a script first, then use the skills you learned from modifying the script into making a totally different one. Also, looking at other people's scripts help. Another thing that you can try to do is decompile a script. That means taking a line (e.g Scene_Menu < Scene_Base) and finding out what it means, before putting it in your own script.
Start of slow, don't expect to make an awesome battle system right off the bat. Some things you might want to try:
Making a script that connects to a Common Event.
Making a script that sends you to a shop, menu, etc.
Getting used to the common classes also helps (Game_Actor, Game_Map, etc).
If all else fails, try looking on the internet for some tutorials. They can be pretty useful.