Buy them with coins earned in the game, but I find it much better to be smart on the transfer market. Browse
www.futhead.com to get an idea of players you want that are in your price range (prices should be added when there's a few weeks of data) and mark some targets. Just on free packs from the web app and buying Spanish players from the proceeds of selling my other guys, I've got this:
Not out of this world, but competitive. Lot of pace down the wings, fairly pacy striker in Ruben Castro and a decent heading striker in Adruiz.
From there, I'll play a few single player seasons to grab some coins then I should have enough players for a second squad and not have to buy fitness cards again as I'll play online with my main squad until they're tired, then switch to the single player B-team. The starters from each team should be the subs/reserves of the other one to allow them to regain fitness.
From there, it'll be all about rolling the dice on packs or marking out upgrades on the transfer market.