After you open your starter packs and being playing the game, you will want to top up your list every now and then with some new cards, which is where you will buy basic packs which are always present in Recruiting.
If you want to spend credits you will get the Local Pack, which mostly contains one- to three-star players plus some equipment, with a small chance for a four- or five-star player.
When you have accumulated a lot of Fame you might want to spend some of on some ring-ins from the Lucky Dip Pack, which has three players of a minimum three-star rating.
Finally, if you have rubies to spare you could go for the big rewards from the National Pack which has the best chance of a five-star card with a guarantee of at least one four-star player. These packs do not contain equipment, but cards can range down to two stars.