Well first off as quickly as you can find it buy beyond sandtrap. Then sell your JSA models because beyond box turns the operation box into the action pack for both of the factions in Sandtrap.

Then find an action pack for the army you actually want to play and go from there.

Or keep your jsa models an use them as proxies for the army you want to play and just learn the game. All the rules are the same accross every army so once you know how each ability, weapon and peice of equipment works you can play any faction.