If you're wanting to collect, just collect what you like. You'll start off with junk or near-junk, you'll discover that things in better condition (cleaner, less rust, with original packaging, with dust jackets, with whatever your field of interest demands) are worth more and you'll find them, and then you'll build a collection worth something.
And even if it's not worth much, so what? -- if you enjoy them! I've bored this list many a time with my collection of old farm and kitchen tools -- butter molds, straight edge razors, plows, scythes, etc -- and probably not a single piece is worth more than two digits, and most are probably worth single digits, but they were things my husband's ancestors used, and they are interesting, and so we like them.
If you're in it to try to make a living out of a hobby, I agree that you should start with what you like, and read valuation books. And keep practicing -- look around antique stores and compare items you see/like with the books. Go to flea markets and fairs -- look, study, read, ponder. Buy when you like something and can afford it. And enjoy it!