This is probably the easiest station, since you hit a few buttons and publish it. However, if you’re IFATC (allowed to control on expert) you cannot open just ATIS.
Before you open any station, Be sure to watch the online tutorials!
I’d recommend starting with just ground and getting a feel for those commands. Learn to pay attention to aircraft doing different things, how and when to use progressive taxi, etc. A lot of these things can be found in the ATC manual.
Once you’re comfortable with ground, move on to tower. Your best bet is to open without approach so you can get a feel for pattern entries and sequencing (If you’re with approach, you have to wait until approach hands them off to you to do anything, so there really isn’t much practice). It’ll also give you a feel for how separation should work.
If you’re comfortable with both ground and tower, go ahead and open both, since you need to be almost perfect in utilizing both frequencies at a busy airport.
After you’ve grown accustomed to doing both ground and tower, you can try out approach. I’d recommend taking it very slowly, starting at smaller airports. In my opinion, the best way to get really good at approach is to join IFATC first.
TL;DR ground, tower, approach
Hope this helps, le me know if you have any questions!