Oh congrats on getting your new pup! You will just love being a Yorkie mom. They are the best.
I don't really have many tips on keeping them from getting sick, if its very very cold I put a coat on my girl and if she's in the rain when she uses the potty I just make sure to dry her off really well once she comes back inside. None of my girls have ever had colds (at least not any that I'm aware of).
Best wishes to you!!!