The best mini selection advice has to be "pick those you like". It would keep you interested and motivated to crack on with them and do the best you can. With a mini you don't care for, you ain't gonna be so keen to work on it, or care so much abou the outcome.
I'd advise a perusal of a general painting advice tutorial. It should inform you about concepts you might not have otherwise thought could be issues. Could follow that up with a stroll through the articles on this site, plenty there to explore and take ones painting into new areas.
Although, the hobby is quite tactile. It could be a good idea to take yourself off to a store with someone installed to show off the techniques. It's a human way of a learning "tell, show, do, check". They'd talk about what needs doing (tell), then (show) it's done, then you (do) it, then can both (check) out the outcome and work from there towards correction or exploring further.