hi jen!
welcome!

dont be scared. you will find that 3-4years will be a nice age for you, they will be very willing to help!
my daughter was 15 months when i had DS so she was a lot younger but i would always make sure she was ok first before doing baby things.
I found that in the early months bub will be happy with a cuddle (slings are great) or a feed - so it is important to have things ready for those moments to make sure your DS feels attended to.
have snacks pre-prepared in the fridge for feed times, i found DD was alwaysw hungry when i sat down to feed!
have activities ready to go so he has nothing to ask for when you are changing a nappy etc - and get him involved! he will love it.

I also found having DH take bub when he got home so i coul dhave 1 on 1 with DD helped too.
xx