Having a baby shower is a great idea! Make sure you give your mum / friend a list of things you need (that way she can secretly spread the word to all the guests)
what we did for a friend's baby shower, was to get her to write a list, then we got a small note pad and wrote one item on each page, let people go through it and just pull out the page of what they wanted to buy - so we had an idea of what was and wasn't being bought for her. A few of us got together and put in money to buy a few "bigger" items (and some on the list that we knew she really wanted but nobody took the page). I did the notebook idea for my 21st and I got the things I really wanted/needed, I'd had the notebook sitting at my mum's place for 2 months before my party - so even interstate friends could ring mum up and ask.

I was also wondering the same thing about when to start buying, not that we're ttc just yet.

GOOD LUCK WITH IT ALL!!!!!