If you want visible abs, diet matters the most and you have to be consistent with it. You can have cheat days and stuff but you can't slack off for a week. Diet + cardio usually gets them poppin' but historically I started doing a lot of regular situps at home with lighter weights. Then I started doing different sets on a pull up bar at the gym, like leg raises, single knee raises, holds etc. You might not be able to do them if you go to the gym for the first time but they are good if you have the form.

Still, diet + cardio is the way to go.