Aah, you want to be a barn slave. I hope you are up to the task. There's a notion amongst barn owners that people should work for well below min. Wage, put in long hard hours and then use time off to pitch in while they traipse off to costly shows. And it seems that those who want to work in barns don't seem to establish ground rules as to your expectations, like paid weekly, on time, without fail, hours of work, extra pay for extras, and time and a half if it's your day off. If you don't do this, owners will have an endless list of "favors" they'll want you doing.
Definitely set ground rules. As far as building a resume/skills - take any opportunity to learn where you currently are! Every day think of something you don't know how to do yet and ask others around you about it, look up articles or even youtube videos for simple things (braiding, wrapping etc) and practice. Our staff pitched in for an online lecture about equine first aid last month. We learned so much!