Allies: I know this whole thing can be scary. It can feel a bit overwhelming, I imagine - It can feel a little bit like you're losing someone you care about. But your friend or loved one isn't becoming a new person; they're still the same person that you've known and loved.
And while you may feel that you have to be strong for the trans person in your life, our allies need support and understanding, also.
Luckily, there are a few good sites just for that.
Trans Road Map http://www.tsroadmap.com/start/tgterms.html
A very in-depth site about all aspects of life with (and as) a trans person. Highly recommended. This particular link is to their General Glossary of Terms, helpful stuff for anyone new to it all.
Straight For Equality: http://www.straightforequality.org/transmaterials
They have a guide for being a good ally and a bunch of other good resources.
Transgender In The Workplace. http://www.hrc.org/resources/transgender-inclusion-in-the-workplace-recommended-policies-and-practices
A good article in The Bustle about explaining transgender to your kids: https://www.bustle.com/articles/109779-5-tips-for-explaining-a-transgender-family-to-your-kid