A general rule of thumb is SGC or PSA for tobacco cards, PSA for other vintage, and Beckett for newer stuff. You'll find plenty of people who have different opinions. In the end, that's all it is. All three have their fans, and you really won't go too far wrong sticking with those three. Also, depending on the cards it may not be worth it as far as doing it for financial gain. Look them all over with a 10x loupe before you send them in and learn what things they look for in that specific issue. Nothing worse than getting your results back and looking at 7's and 8's on cards from the 80's or 90's to today. That $10 or so a card you spend to get it graded is tough to recoup when you get less than a 9, so be sure your cards are that good before you send them in.