The best advice for making a happy marble collector is this: Collect what you like.
Nobody enters this hobby knowing exactly what he likes. Given the variety of marble makers, styles and eras to choose from, even veteran collectors stumble upon styles or examples new to them. That's part of the magic of collecting marbles.
Finding out what you like can take a very long time if you are just finding handfuls or marbles here and there at garage sales and flea markets. It is a fun and exciting method for discovery, but it can be slow and frustrating depending upon where you live and how lucky you are. Worse, you typically find marbles in poor condition.
Instead, I would recommend you attend as many marble shows as you can, where you can see and handle the real deal. Marble collectors love to talk about marbles, and most eagerly tutor beginners. They get a glint in their eye and will enthusiastically bend your ear on marble colors, condition, seams, and patterns, and other minutiae. You can learn a lot just going from table to table asking questions, and you're going to see great marbles in great condition. When something catches your eye, ask everything you can about it. Purchase the best example you can afford, and take it around to other tables, show it to them, and ask more questions.
By the end of one marble show you should have a better sense of what you like, and hopefully a few nice marbles to show for it. It will also lower your tolerance for marbles in poor condition, so you don't spend more than you should.
Soon you'll discover just what you like, rather than following popular opinion. Then just rinse and repeat.