Go backwards when coming off the bus! That's is honestly the best and main advice I can give . Other than that , take advantage of the fact that you don't drive. You guys are going to have so much active adventures/fun memories. You will be ok my dear x
Definitely see if someone can do a trial run with you to build your confidence. I try to avoid before 9am and after about 3ish because it gets busy but if you get on at the first stop then it's not really a problem getting on. Generally bus drivers are nice and patient when you have a push chair x
The best thing to do is journey plan. Check your route door to door - so you can estimate the journey time. Then work out your bus times and when it’s busy etc - so avoiding rush hour/school time so there’s more likelihood of space on the bus. I’d have a carrier with you just in case you do need to carry baby and fold but most cases if there’s no wheelchair occupying the space - there’s space to get buggy on. Have you and your partner thought about going on the bus together as a trial run for the first time and do a short ride to get a feel of it?