Collect something that interests you. I love collecting ancient Roman coins because I learn so much about the history of the time when I research each new purchase. I also collect fossils, minerals, meteorites and old comic books. But pick whatever interests YOU. Start off buying the cheaper examples until you learn more about price and value - the rarest, most expensive types will tend to be the best investments, but save those until you're sure you know what you're doing!
If your thinking of buying a collectable and its marked up as "RARE", the chaces are its not Rare at all. Most mass produced items ( Records, Toys, China, Books, Comic's, Coins, computor games etc ) were manufactured and sold in there Thousands and in a lot of cases MILLIONS all over the world. They may be hard to find but THEY ARE OUT THERE. Just trawl through ebay and see numerous identical "RARE" items listed on the same day, with prices ranging from cheap to reasonable to... outragious (usually by people "trying it on") Buyer BEWARE scammers are out there. GOOD LUCK. T.J.UK :)
Love to go to estate sales and garage sales just for the fun of seeing what is being sold for cheap when my intuition says "there's value hidden in that." Checking comparative prices online can sometimes be exhausting, but I found an intriguing book among the two boxes full I bought for $20. Called "Angels and Women" I couldn't find it anywhere on the Internet except as a new paperback. And I discovered that I could become a professional bookseller on Amazon! So I did, made up a price for the book, and listed it...5 weeks later I sold the book for $879.50. I now have over 5,000 books listed. Best tip: do what seems like fun. I'm having fun...
Maybe a wise idea for collectables is choosing items that grow with age, coins, stamps, etc. Go into antique shops and see what is always being asked about. Keep your collectables cared for and in a safe place. Some can always be set out for others to see but not the more expensive ones.