To buy more card packs, you'll need to use the money you earn by winning games and tournaments in FIFA 15, though you can also use real money. In the beginning, you'll receive several packs of cards in which you will find players, contracts, coaches, training systems, and all the usual stuff from 'Ultimate Team' mode.
With FIFA 15 Ultimate Team, you can put together the team of your dreams from scratch. FIFA 15 Ultimate Team is the annual installment of the FIFA series for Android devices, which, as usual, allows you to enjoy the 'Ultimate Team' game mode from your phone or tablet – and it's completely free.