If you're going to get one tool to mod with get the dremel, get a good one and you won't regret it. After the dremel it depends more on what kind of modding you want to do which will dictate what other tools you want. However, you can do just about anything with a dremel given enough patience and practice.
If you're cutting metal, get the metal cut off discs, I recommend the reinforced cut off discs, they are a bit more expensive but the make faster and cleaner cuts. I find they also last longer therefore making up the difference in price:
Homedepot.ca - Search - dremel cut
I use the EZ Lock cut off discs, they are just faster to swap in and out than the ordinary ones. Both are great.
Best thing to practice on and still have a productive result is old cases, cruise your kijiji and craigslist classifieds for people dumping old cases for like $5 bucks. Mess around on those for a while practicing your cuts etc...also good for practicing spray painting, bondo work, etc.... and in the end you have a finished product rather than just some cut off scrap materials.
Other cheap tools I use lots of: $4 set of needle files for getting rid of the burs lift by the dremel set. I use a hole saw a lot but they are expensive so I'd stick with the dremel for now. A drill is key, same with a tap and die set...but again that can come later. Mouse sander for painting prep, also use a wire brush attachment on the dremel for this. And so much more....best way to learn is read other mod logs.
Great place for tips on modding cases is to check out mnpctech.com and the forum themodbrothers.com much smaller venue than this forum and they don't really touch on hardware stuff much but instead focus on modding. Good bunch of guys for anyone, very noob friendly with lots of constructive feedback and tutorials.
Other than that once you get to a specific task you want to do then post technique questions on that...for example cutting a 120mm hole with the dremel. It's easier to give tips on a specific task than modding in general, considering it encompasses so much.