IMO, having cards slabbed is completely useless, unless we are talking vintage cards or early to mid-90s foilboard or chrome-type cards (ie. SP, Finest, Topps/Bowman Chrome, Pacific Prisms, etc.). As for having cards graded, the top two graders are easily BGS and PSA. Those are the most wanted and respected graders, even though both have had issues in the past. They are both still better than any of the other companies grading cards.
As for what to submit, I'd recommend ONLY high value or vintage cards. If the grading price is higher than the card value, you're losing money even if it's for your PC. You say you're a newbie and I don't know if that means "to the site" or "to the hobby", but Vintage is anything made before 1980. I have never submitted cards for grading b/c I can't justify spending the money for that when I can protect cards just as well with UltraPro cases inside a team bag. I have thought about having a few cards graded though. My short-list of cards I think about getting graded are:
1979 Topps Ozzie Smith RC (probably an 8 grade or higher)
1909-11 T206 # 277 Bill Lattimore (probably a 2 grade)
1971 Topps Joe Namath #250 (probably a 4 grade)
1983 Topps Ryne Sandberg #83 RC (easily a 7+ grade)
1984 Donruss Don Mattingly #248 RC (also easily a 7+ grade)
1992 Bowman Manny Ramirez RC (maybe a 9 grade)
I wouldn't even bother for cards that have serial numbers lower than about 50 copies. The number of copies is low and the grade isn't going to help you sell it. Even if it is a jersey-numbered/door-numbered card, it won't change what someone is willing to pay.
I hope all of this helps you figure out what you want to do with your cards!
NO PWE's EVER!!! PLZ PM me 1st before sending any offer. ONLY selling cards as of March 2024. No trades or purchases right now.
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Largest total PC card collections by Team, then Athlete (as of 3/22/24):
STL Cardinals (MLB) - 8810; Carolina Panthers - 2888; GB Packers - 1790+ cards
Mark Martin (NASCAR) - 2038 cards; Jimmie Johnson (NASCAR) - 1875 cards; Jeff Gordon (NASCAR) - 1594; Ricky Rudd (NASCAR) - 839; Ozzie Smith (MLB) - 707