It's called a "condom". After you have ejaculated you need to throw it away and if you are still erect and ready for more sex, you need to slip on a new one. If you keep using the same one there's a chance it could slip off and not work effectively. The condom shouldn't be too tight. There should be abit of space near the tip of the penis so it can act as a sperm resevoir and wont break open. Sizes are mostly standard or large.