Collecting signed memorabilia from the most famous athletes in the world can be a financially rewarding endeavor, this shouldn't be your first priority. If you are new to the sports memorabilia.
The most important thing should always be, actually enjoying what you're doing. This may seem pretty obvious, but many people start collecting simply because it could lead to making some money. Usually it doesn't work that way.
People that get into this business just strictly for financial gain, usually fail, and quite badly too. A true collector should really appreciate he or she gets. If you are ever lucky enough to come across a Mickey Mantle autographed baseball or a Lou Gehrig signed jersey, the first thing that should come to your mind is not "how much could I get for this?". The first thought that should come to your mind is "Wow! I can't believe I'm lucky enough to own this". That piece of sports memorabilia should be viewed as an unbelievable piece of history.
It's no different than an art collector buying a Picasso painting just for the incredible privilege of enjoying its legacy and what that painting represents.
Another important factor to know about collecting sports memorabilia is that there are a lot of crooks out there. It's not too hard to take a pen and fake an autograph from Joe Montana or Kobe Bryant. Unfortunately for all the true collectors, there are many shady people that have created an art form in forging athlete's signatures.
Thankfully though, there are many authenticating services who have an excellent reputation. There is PSA/DNA, Steiner Sports, Upper Deck, and many more. The sole purpose of these companies is to identify that an autograph was actually signed by the respective athlete.
Although these authentication services do the best they can, there is always a chance that one gets past them. One way to get remedy this, is to actually get the autograph yourself. If you lucky enough to live near a professional sports team in your area, just take a drive down there. After practice, most athletes spend a good 20 minutes just signing different things from fans.
Just bring a baseball, football, poster, jersey, bat, card, or anything else you can get your hands on. Try to remember your pen. You'd be quite amazed at what you end up leaving with. And the most important thing is that you know that the signature is 100% authentic.
If you want to start buying your memorabilia, there are a lot of quality online autograph sellers. I also recommend going to eBay. I know some frown upon eBay, as they do have some scammers, but the majority of the sellers are very respected in the autograph industry. Also, you just can't beat the selection. Obviously, the important things to look at before a purchase is:
1) Is it authenticated? The majority of the sellers show their certificate of authenticity on the listing.
2) Take a look at the signature on the piece. For example, if you're looking at a Reggie Jackson autographed bat, do some searching online. You're bound to run into more Reggie Jackson autographs. Just simply compare and contrast the ones you've seen to the one that is in the listing.
3) Also most sellers have refund policy. So get a second opinion on your autograph. Ship it to another authenticating service. If it comes up as a fake, most sellers will take it back for a refund.
4) Most importantly, check the seller's feedback. That's what it's there for. If you see some unhappy buyers, you know this seller is not for you. If you see nothing but positive reviews, then you know the seller is certainly more trustworthy.