I'm sort of a beginner. I've done it about three times now with a man. (very young the first time with my best friend) The first guy was a big man with a medium sized dick. Hard as blue steel with a big head. Just long enough to choke me if I pressed my face in. I actually find it erotic to do that, and he was really gentle and understanding so I was able to without having to deal with being forced. It was a great experience!
Our first meeting was only supposed to be a JO if I was comfortable. Perhaps more but neither of us was sure. He was a receiver only which was a big plus for me. Switching off is a pain when you get going. Anyway, best advice I can give you (as others have) is go slow. Be very very sure you aren't taking a risk. Most guys won't have a problem with wearing a rubber if you insist, and won't mind warning you when they are going to squirt so you can get off. Personally the experience is not worth it if I can't feel the flesh and taste the result. Again I've only done this a couple of times and was very careful of partners. Mostly older married guys who have something to lose (family etc) seem safest. Remember all men lie. (I got this from a TS lady I know. She's right)
Don't be afraid to ask if he has STDs. Normally I get "not as far as I know..." and so I follow up with a couple of loosely shrouded questions to find out how active and/or risky he is. I almost went for some guy in a book store once. Usually I just peek, but this time it was there so I figured I'd give it a try. It smelled like lube and something else (dirty) which just grossed me out so I said "sorry. no" and walked out. If it doesn't feel right, it's probably wrong. Don't be afraid to say NO especially if the guy is a pig. Remember you're giving him pleasure. You're in charge of what happens, and you carry with you anything that he passes along.
Something else I've learned (with no real substantiation) is don't floss or brush around the gums before receiving a load. Better to leave the plaque there as a sealer and brush afterwards. You can always eat parsley or mouthwash to clean your breath or brush the teeth carefully (without hitting the gums). The chance of getting something orally is small from sperm(if you spit). Tiny (almost non-existent) from pre-cum. Still play safe and you should be ok.
From the up side, it's an amazing feeling having another man's penis sliding along your lips and tongue. The thrill of having your mouth full of that hot member is beyond description. It's invasive as hell and extremely erotic. Especially the head! I will also tell you this from my limited experience. It's EXTREMELY addictive. The more cock you suck, the more you want. I've had two guys shoot their load in my mouth. One I expected and it was nice. The other was not expected or desired, but I have to admit it was amazing. The taste and warmth as it rolled across my tongue was dizzying. If I was sure of this guy I most certainly would have made this the first load I swallowed, but I wasn't and it was a full bottle of spring water and half a bottle of listerine before I finally gave up trying. Nothing happened, but I was still terrified for months afterwards. Don't go to the ABS loaded.
Oh, and no, sucking cock didn't make me ramble on and on. That's a trait I've always had trouble with.
good luck bud.