My advice is to learn as much as you can through books, websites, articles, and daily practice. Don't limit yourself to just makeup application- get interested in fashion, photography, product ingredients, skin, etc. They are all interconnected and will help shape your good taste and your eye. Then start putting together test shoots in order to build more knowledge and experience. You will start to understand how makeup photographs and what is proper set etiquette. Do as many faces as you can, because that's how you will learn the fastest. Makeup artists don't only apply makeup to llamas- it's important to know how to do makeup on non-models, for events for example.
Later, the mentoring or assisting can teach you whatever you were not able to learn on your own. You want to be prepared when you assist an artist- you can't expect them to baby you through your career, if that makes sense.
Finally, in my opinion, your money should be spend towards building a strong kit. Here again, research will help your save time and money.