Upgrades in WoT are accessed from in the garage, where you can see all your tanks at the bottom.
while highlighting a tank, then choose the "Packages" option. In this menu, buy any upgrade.
Do remember--it takes tank XP to research upgrades, and then silver to buy them. You get both by playing matches. If you do the training, you should have some silver also.
Also, since all Tier 1 tanks have all upgrades already researched and bought, you will need to buy any Tier 2 tank, and then research and buy one of its upgrades.