I'm really into having girls sit on my face from time to time. I'm always a little disappointed that there seems to be a lot of hang-ups about it, because it's very fun for me. It seems like there are three main things that come up as concerns.
Weight/suffocation. You are seriously unlikely to hurt your boyfriend by sitting on his face. Even if you're extremely large, it's just not something that I'd worry too seriously about. The worst issue I've had was a little bit of a bloody nose when a girl really started bucking against me and I caught some pelvic bone to the nose, but nothing terribly serious has ever happened. As a general safety rule, keep at least one airway clear. It's not too hard to breath through your mouth while eating pussy in that position, but just on the off-chance try to keep his nose clear. Additionally, try to make sure most of your weight is distributed on your knees rather than, say, his neck or chest.
Hygiene. Chances are, if he's eating your pussy on the regular (and is enthusiastic enough about it to ask you to sit on his face) this isn't an issue for you. This is really a no-brainer, just keep yourself clean. Since you said you'll be sitting forward, the whole anal hygiene thing isn't as big of an issue but that's also pretty much common sense--wash your ass and you'll be good to go if you do end up sitting reverse (which I actually prefer, I enjoy the view and I feel like I get better access with my mouth, but that's another story). A well-washed ass doesn't really smell, but, I mean, you can't blame people for being a bit freaked out I guess.
The "What do I even do?" question. Honestly, the simple answer is enjoy the ride. Your boyfriend will be focused on you, and honestly he's not likely going to likely be paying a whole lot of attention to most other things. If you're feeling ambitious, you could reach back and stroke his cock with your hand. I also like when a girl sorta thrusts her hips back and forth a little bit. If you turn around, you could 69 with him which is always fun. If you're feeling particularly playful you might even tug at his hair a little bit.
General advice: stick a pillow under his head so that it's propped up just slightly and if you have week knees brace your arms against the wall or the headboard.