I've noticed I got the 7th Dragon Ball but I don't remember how I got it. What's the way to get all the seven Dragon Balls?

The dragonball you already have is probably from story mode, once you complete it you get 1 dragonball.
To get all seven dragonballs you have to buy each one at Mr. Popo's shop for 8000 zenie each. They will be in the recommended capsule section, each time you exit the shop and re-enter a new item will be in the recommended slot, eventually each dragonball will show up there. Another way is to buy the gamble capsules, the dragonballs will sometimes show up there also. Once you have all seven go to the skills or caps section and select the dragonballs, you can then summon shenron and he will give you a breakthrough capsule for one of three random characters. The capsule allows you to use all of that characters moves in a match at full power or level but no items. If you don't like the characters that show up press restart before you choose anyone and go in again, a new set of three characters should show up. Once you have summoned him you have to collect the dragonballs again to get another capsule...