[17M] I think I’m being taken advantage of and I don’t know what to do

Ok, so I’ve been working in a cafe for 9 months, and the thing is, I’ve never, not even once, been paid when I’m supposed to. It’s frustrating because all of my co-workers have been getting paid at the end of the month, whereas I don’t, and it gets to around a week or so later when I decide to message my boss about it, that I get some lame ass excuse like it’s “processing” and then goes into my bank account 5 minutes later… It’s my first ever job too, whilst I’m doing my A-levels, so it’s all new to me. Also, I don’t think I’m actually being paid the right amount. I’ve been working it out, and it’s a lot lower than it should be. It means that I’m earning way below minimum wage, and I’m actually shocked. I haven’t been feeling great mentally for a long time now, which is obvious with other things I’ve posted in other subreddits, but I don’t know, what should I do?