They were two sisters who were shopping at the mall.

Avoid word repetition unless the words after are interchangeable. Mostly though the 'who' pronoun isn't doing anything here even if it's okay.

Julia tell"Look, those dresses, don't you think they would look great on me?" Julia says to Anna:.

There are some rules for writing like this, I don't know them perfectly. Also, "do" is actually not correct here, everyone uses "don't" it being a rhetorical question, or otherwise one to be sure is given reassurance vs via a logical step removed with do..

"Anna: absolutely, what's more, they match your skin tone." Anna replies.

Julia: continues "But what about if I try them on?"

Anna: okay"Well, let's do it." Anna decides.

Julia:"Okay great they are my size, I'll go to pay for them." Julia says finishing up.

I think go is a similar word as to, they both are close with 'towards'. You don't need both.

Julia looks around for the salesperson, and say to him : eupon finding him says "Excuse me sir, I'm interested in thisese dresses. I'd like to buy them., okay?"

Salesperson: g"Great, a good choice " The salesperson replies.

He continues, "These weare the latest, and by the way, they're on sale."

"They'll cost you $150 total is one-hundred and fifty dollars." He suggests after working the register.

Julia: is shocked, "I don't believe it!, thanks you sir, and have a good luck with your sale.s."

Anna yand Julia go shopping.

Theyre were two sisters who were shopping at the mall.

Julia tell to Anna:

Another option is "Julia says to Anna,".

Look at those dresses, don't you think they'd look great on me?

Anna: aAbsolutely, and what's more, they match your skin tone.

Julia: whatHow about ifI try them on?

Anna: oOkay, let's do it.

Julia: gGreat, they are my size, I'll go to pay for them.

Another option instead of "they are my size" is "they fit me".

Julia looks for the salesperson and says to him : e, "Excuse me sir, I'm interested in thisese dresses. I'd like to buy them."

Salesperson: gGreat, good choice .

These weare the latest, and by the way, they're on sale.

Julia: I don't believe it!, t Thanks sir, and have a good sale.

"have a good sale" is grammatically correct, but not a statement I've heard said much to salespeople, so I removed it to sound more natural.