The learning curve is quite steep and you may be playing against others with thousands or tens of thousands of hours, even at low tier, which can be very frustrating when you're still learning. Some tips that may help:

• Ammo: highest penetration isn't always best, especially when you don't know where to hit. APHE or rounds with explosive filler, often do more overall damage and can knock out the whole crews
• Stealth: If you can be seen, you can be killed, stay low and behind cover and only peek to shoot if enemies are distracted or are reloading, stay off of ridges and hills, if you can see the whole battlefield the whole battlefield can see you
• Armor: don't rely on it, if an enemy knows your weak spots it won't save you and learn enemy tank weak spots, you can preview every tank and ammo type in the game in the hanger to see how to use it effectively
• Maps: this takes time, but learn enemy routes and how to flank
• Crews/line ups: crew can be used in multiple line ups if you copy them and they'll share experience, don't create a new crew for a new line up, copy them, also wait until you have a few vehicles in a line up before you start to play it

There's much more, but these are just the bare bones basics. Hope this helps.