You need 1 first class international forever stamp which costs $1.15. That covers the first ounce. If the letter weighs more than one ounce, you need 21 cents worth of postage for each additional ounce. If you don't have an international forever stamp, you could use 3 domestic forever stamps which are 49 cents a piece.
How many stamps do you need for an international letter?
International First-Class mail costs about $1.15 presently (Reference: First-Class Mail International). Each forever stamp currently is valued at $0.47. Therefore, you can use about three forever stamps to send your letter/post-card/etc. internationally (Reference: Forever Stamp Fact Sheet).
How much is postage to Europe?
USPS Postage Rates
|New Rate (effective Sunday, January 21, 2018)||Postage Rates|
|First Class Mail Flat (1 oz.)||$1.00|
|First Class Mail Flat - each additional ounce||$0.21|
|First Class Mail International Letter||$1.15|