Quoted from DerGoetz:World of Tanks is free
You can get a premium account in order to grind credits and experience faster. But absolutely not needed in order to have an absolute BLAST
WoT is great! I just started playing in August and usually put in an hour or so each night. You have to put some thought into equipping your tanks and crew skills, and definitely some stragedy to the different tiers and maps. Good fun!