What do I do if I get a sub gift?

I know that you get sub gifts by chance, but I wanted to ask how exactly it works. If you happen to get a sub, you have to press something or confirm something or are you just sub?

would be very nice if someone can help me because i only have twitch a few days and understand little about it

You just have the sub. If you are in the stream and get one, you can also press a pop up to say thank you, but that is not necessary to get the sub.