Welcome, sadieme. You are right - this is a great forum!
While I don't post often, I have learned so much from everyone here - and not just about cycling!
About gears & shifting - I remember asking the same things 15 or so years ago when I started riding again. A cycling friend told "you'll just know"! Didn't have a clue at the time as to what she was talking about - my thought was 'what the heck is that supposed to mean?" Guess there is some truth to that because I did learn what felt right! As someone else said earlier, shift down before you have to. You can always go into a higher gear but is it tough to shift down when you are struggling on a climb!
And no chance that 55 is old! I'm 53 and I think there are quite a few of us 50-something ladies on TE!
Serendipity
"So far, this is the oldest I've ever been....."