How do I start stealth training? Where do I find that girl?
In order to get the stealth training you need to have at least done the following.
1) Started Maria's chain with getting her her package,
2) Get you room key from her room after making a copy of her room key.
Once you done these you just go into her room during the dawning (To do this you need to wait until 00:00 or set your alarm before going to sleep. interact with her and try to move her and from here she will wake up and it will say you need to improve your stealth.
The next morning when you leave the house you will run into the thief girl who will also be your stealth teacher. once she steals money from you it will mention going to east town. However don't go there as of yet. wait until it becomes evening or dawning before you go. Once this time happens go to east town, and go to the alley at the the most southern east part and she will be there waiting for you.
Speak with her and in exchange for getting Wagner's packages she will give you a stealth mini game. as for the packages themselves there are 25 in all and they can only be found in certain sports at certain time of the day but have unlimited chances to get them. The exceptions to the rule are the following
1) The warren missing package (You can only get this one while doing this mission chain and you only get one chance to get it. If you miss you can never get this one again)
2) The Derek package (within his apartment when you are getting the evidence to prove he is cheating on your stepsister like the former you only get one shot at this one, and if you take the picture with Chloe before getting it you can never get this one again.