Can I use "she" instead of "her" here?

Since they have similar meaning, can I simply use "see" instead of "look at"?

Yes, in this context they can both be used. However, it's a bit redundant to use the same verb twice in such a short sentence, so the speaker has used both "look" and "see" here for variety. Ex: Look at that ship over there! Ex: Do you see the ship over there?

What's "ever" mean here?

Here, "ever" means at any time or point. It emphasizes the comparison between now and any other possible time. Ex: Right now, I'm happier than I could ever be. = Right now, I'm happier than I could be at any other point in time. Ex: We're spending more now than ever on take-out.

What does it mean when someone says "that's the last thing I want/need"?

It means that they really don't want or need it! Out of all the possible things they could want or experience, that would be the very last thing they want. It would be the worst thing possible. Ex: Since I'm writing my exam tomorrow, the last thing I need is to be late for school. Ex: I'm so tired, the last thing I want to do is clean my house. I want to sleep!

What does "audience" mean?

An "audience" is a group of people who watch a performance, such as a concert, play, movie, etc. At a concert, the "audience" can also be referred to as "the crowd". Ex: The audience screamed throughout the entire concert. Ex: The actors bowed facing the audience at the end of the play.

What does it mean to make yourself a brand?

"Make yourself a brand" here means "create your own brand". The speaker is telling one of the other girls that she should start her own brand, possibly because she can't find a brand she wants to work for.