Can't stress this enough - get your own account and not a used one (if you haven't already), it will save you a lot of bother and grief from the previous owner in terms of bans and general reputation, not that we are all bad
As for payment, follow various pieces of advice given. I'd say PayPal is ideal, as for age restrictions...Im sure your mum is old enough to register one.
After showing her this forum and thread, and that privacy statement, she still doesn't feel it safe enough, perhaps update her on the workings of the internet and virus software, etc, etc.
Josh.