This is a pretty fundamental part of World of Tanks but it is important to remember one simple rule when it comes to upgrades. Always go for the tracks and best gun first! The gun is your ticket to earning experience and credits which you can use to buy and install the rest of your upgrades. Without a decent gun, you won’t be able to penetrate some of the tanks you meet which hampers your income. However, because newer guns are often heavier they usually require upgraded tracks, which have a higher load limit, so it is usually a good idea to do this first.
Also bear in mind that sometimes you do not need to buy every upgrade for a vehicle. If the improved engine only gives you 20 extra horsepower or the new gun is exactly the same as your old one except with 10mm more penetration, then it might not be worth the cost.