There are 3 sets of dragon balls. If you collect all 7 dragon balls on a set you can make a wish.You need a Dragon Radar(5000 zeni on earth shop) to search.
Here are the wishes you can make.
1.You can wish someone back to life.
2.You can wish to know any techniqe.
3.You can wish to have any item.
4.You can wish to have up to 25 000 zeni.
5.You can wish to unlock his/her sleeping powers (gives 20,000 ki).
6.You can wish to own your own planet.
You can search for them on earth. You can have 1 of the wishes from on the top and you can wish back unlimited people.
You can search for them on namek. You can have 3 of the wishes from on the top, but you can only wish back 1 person per wish.
1st Star=Mirai goten
2nd Star=Mirai goten
3rd Star=Mirai goten
5th Star=Mirai goten
6th Star=Mirai goten
7th Star=Mirai goten
You can search for them all over the galaxy. You can have 1 wish from the top. and you can wish for up to 50 000 zeni on this one.
When searching for dragon balls you may also stumble across unique items(we will tell you if you do), you can sell it to the pawn shop, just ask vegeta about the item and he wil tell you. You can also identify it at the pawn shop for 1000 zeni and yo may be able to use it deending on what you find.