Tips and tricks for new players?

Newbie here started plating recently. I do fine (70 ish percent win rate) when I sta below tier 5 but as soon as I hit tier six I just get destroyed. I play with a friend who's also new. Does anyone have any tricks or tips for a new player, as well as things to absolutely *not* do, and conversely things I absolutely *should* do? And why does the game feel so much different after tier 6 tanks?