Help me it sais i have to "open" with it

Hello, when I try to open lets say it makes me choose a program normally i have no problem with this but when i have to download something i have to open with something and it comes up with different numbers and symbols and i don't know how to download it correctly. but when i bought the computer it never had this problem and i don't know when it like this can some one please help me. if you need any information just ask because your helping me.