It's not fair to treat me like that

I've been working for a few weeks and LATELY there is this one girl who bugs me and pick on me about how my shirt is and me being lazy and not having confidence. Let me break this down easily as possible.

- How my shirt is..
~~ My shirt I wear is black and it's also a dress shirt. It has to be a dress shirt and tucked into my dress pants. So my found out my neck line is to deep..  She [Sarah] told me that I'm showing too much. Which I wasn't because she was the one with the hanging out cleavage practically her boobs falling out. Mine was just close but not fully button. You see it's hard to find those really good black dress shirts that buttons all the way up and gives a little to BREATH. When I talked to my manager she told me it wasn't CPK approved. So I was ok with my manager telling me since I talk to her. She also said that Sarah wasn't suppose to talk to me and rationally pick on me and should of went and talk to the manager first.

- Being lazy..
~~ She picks on me just because I don't sit customers down. Which I do not know the rotation. Plus I'm new and she expects me to know everything. I do not know the rotation and how to sit people and when I sat people the wrong place I get scold at which wasn't my fault. We have hosts to sit people not servers.

- Not having confidence..
~~ I do have confidence. I don't want to mess stuff up. Plus people are work speak spanish so its hard for me to hear them or talk to them. And once I do they end up asking me a second time so they can understand. I do have confidence but I'm new to the working industry so what do you expect me to do? I only JUST started and it has only been a month. And I just started serving 2 weeks ago.

Advice?
-- TALK to your manager and let them know what's going on because it's call discrimination no matter how you put it. If you feel emotionally attack and no physical what so ever it's still discrimination.