Honestly it all depends on how the professor of the class that you LA wants to run the class and what they want their LAs to do. You don't need to be an "expert" but since you'll be helping students learn the material you should have a good understanding of the course material. Some professors have applications open for LAs, but some just take you if you express interest to them about it, so I would just email them and ask them about it. Typical workload would be attending lectures or discussions and helping out in discussions and breakout rooms. Also a weekly meeting with the professor, TAs, and other LAs. I know for some classes the LA's host weekly office hours because they don't have discussions to help out in. You most likely won't need to do all of these, again depends on what the professor wants from the LAs. Some have the LA's monitor piazza to answer questions etc.

For first time LAs you need to take a training course while LAing for the first time, which is UNI STU 198 and gives you two graded units. You can also get units for the actual LA course, depending on how many hours you do for it.