Researching new tanks requires two things: experience and credits. To get a new tank in your Tech Tree, you must upgrade your tank (in the direction of the tank you want) by buying required items that allow you to progress toward the desired tank. For example, if you had the U.K.’s Cruiser I tank and wanted to upgrade to the Cruiser II, you would open the Cruiser I’s Display Modules and buy the Suspension A9 Mk. I.
You can then buy the Cruiser II after you have purchased that upgrade. On the other hand, you could also pay gold to instantly upgrade your tank to unlock the next one.