Luckily, I'm an extremely helpful and knowledgable player who tells it like it is.
Here is my answer:
The reason one friend told you that Shermans are good and another friend told you that Shermans are trash is because there is no one size fits all tank.
Both of those players could be correct, and neither is wrong, because they are basing their answers on how they do in that tank and how they like the tank.
Different players have different play styles, so it's fully possible for two different players to have different opinions on a certain tank.
Some players like to drive lights, some players like mediums, some players like heavies, some like TDs.
Here's the deal.
If you've only been playing for less than a week, then you obviously have a lot to learn, a whole lot.
But the fact that you came to the forum and asked this question after only playing for a week shows that you are smarter than the vast majority of nubs that play this game, so there is hope for you.
There is no easy answer or easy solution for you or for anybody else who is a complete beginner. You will simply have to get to know different tanks for yourself, and as you get better you will start to know which tanks you like and what kind of tanks suits your play style and your preferences.
And no, Russian tanks are not trash. Some of them are the strongest in their tiers. Don't listen to that friend anymore.
Drive what you currently have. Eventually you will buy new tanks. Some of them you will end up liking. Some of them you will not. The only way to know that is to actually drive them for yourself.
You have a lot of experimenting to do. There are no easy answers. There are no easy solutions. It will take you a long time before you will become experienced at this game.
You will regret some tanks that you buy and you will like others. You will die a lot. You will lose a lot. If you're lucky, you'll kill some enemies along the way too and win sometimes too.
The most important component in this game is not the tank, but it is the player.
Good luck.