i'm gonna guess it's an iphone since you're a handicap and didn't tell us what phone you have.
go to settings, go to wifi, and make sure that stuff is on firstly. second, make sure "Ask to Join Networks" is set to ON. if you join a network, it automatically sets it as a "known network". especially if you connect to it more than once. now just make sure that you have the ask to join on still, and it will automatically join that wifi. works even better if the wifi you're joining has a password.
