You don't need to say anything to the bus driver: just give him a pound coin or silver coins adding up to a pound (or a £2 coin and get your change) and off you'll go! Main campus is the first stop from Egham Station. The bus then proceeds onto some of the residences outside of the mainn campus.
I've no idea how many undergrads commute. It's common for postgrads to commute but I did my undergrad and Masters degrees elsewhere, so I'm afraid I have no idea about undergrads
Don't be nervous - remember loads of people won't know each other and I'm sure many Londoners will commute from home. So there will be people in the same boat as you.
During Welcome Week there are people wandering around campus, usually in orange T-shirts/raincoats to help you. So don't hesitate to ask them anything/everything! That's what they're there for
The campus is relatively straightforward to navigate once you've figured out where Bedford Library (the social sciences and sciences library) is. It's hidden away but once you've been there once or twice, it's all straightforward
Good luck with everything and enjoy Welcome Week
Do post again if you have more questions and I'll try and answer. Can't tell you everything though, coz I'm a different level of study, etc.
Btw, I recommend downloading the CampusMe and UpayChilli apps if you have a smartphone. The former is a RHUL app that has a map, telling you where things are. You can also check the library system on it, etc. The latter app you'll need to top up your campus card with money for the cafes, etc. (which I recommend doing - if you pay with your campus card, you don't pay VAT!). There are machines on campus that you can top up your campus card with but there aren't that many of them. So it's easier to top up online with UpayChilli, on the go