Hi TheCollector1992,
Well, first of all, welcome to fanmail.biz and to the wonderful world of autograph collecting!

It's great to have you here, this is an excellent hobby but it does require quite a bit of patience..
What you need to include in your envelope? I'd check out
THIS and
THIS OTHER ONE for some good basic autograph collecting information about sending requests and what to include in your envelope. As a basic guide, you need your letter of request, a SAE (self-addressed envelope), IRC (International Reply Coupon) the number of these depends on what size envelope you are using as an SAE. And optionally, you could include a photograph for the celebrity to sign.
You put International Reply Coupons in Your envelope for the celebrity to take it to their post office to exchange it for stamps.

Don't put it on the envelope as you won't get anything back! :razz:
It costs $2.20 to send international letters in Australia, weighing up to 50grams and can be used for large envelopes. I usually use C5 size envelopes as they fit 6x8 size photos and I usually only need one stamp.

So, a $2.20 stamp would be fine for sending to the US or UK.
If you need any more help, please just ask here or feel free to PM me.

Olivia