The USPS website will tell you exact prices. Worth noting that a board backed envelope will cost more. Mine usually need around $3 of postage on them.

From the US to the UK (i assume it'd be the same visa versa) to send a letter sized enevelope, you need an international stamp which costs just under a dollar. I get them from the post office. I'm not sure about a big sized envelope, though. Stop by your local post office, they should have the information. :) Or try googling it.

Hi,

I am really new to collecting and would really like to write to get some american actors signatures, how much postage should i use and where would i get american stamps from?

Thanks