That's a huge question! What version of the game do you have?What mod(s0 do you have installed?What mods do you want to install???
Right now the latest mod is the Ultimate mod that was released at the beginning of the year along with monthly small patches( the latest one is dated June 2012) tht add new vehicles, features, improvements etc....
For the Ultimate( the link is the top sticky called Ultimate Mod released:
http://graviteam.com/forum/index.php?topic=10117.0.There you'll find the links and the explanation on how to to install mods and missions.
For Ultimate to work you need to have the Steel Panzer Beta mod 1.5 installed and enabled in your JSMGE. Here's the link for the English-language version:
http://files.mail.ru/VSM7BV in that same thread you can read about the mods installation order.
Another site I use for mods and downloads is a Russian one with English language releases here it is:
http://tanksim.org.ru/modules/wfdownloads/viewcat.php?cid=15My advice is to take time going back of few months to read the various posts and threads on the fourm.It's a bit of a pain but like studying ins hcool so you can pass your exams it's essential, interesting and helpful.It alos puts oyu in the loop and allow you to gain a lot of useful knowledge...