Hi Lara,
I am new to minis too (we showed QH for 20+ years), so I am sure you will get some great info from much more experienced competitors than I am with the minis, but I would suggest that you maybe look at videos on youtube? (That is what I have been doing.) If you put AMHR or AMHA show in the search function, it will bring up show videos so you can see what types of preparation you will need for you and your horse, as well as show you all of the classes, and give you an idea of what is offered and the levels of competition.
Also, go to shows and watch if you have any in your area you can attend. Doing that gave me a very good idea of the types of classes I thought I'd have an interest in.
If you have a trainer in your area, maybe see if you can work with them for a "mini clinic" where they can come to your place or you can go to theirs for a day or two and learn all about what kinds of classes you might want to go in and how to prepare yourself and your horse.
Good luck!