you approach a girl and you say "don't wear that shirt"
she says "why not?"
you say "because I don't like it"

I have tried this a few times, and probably due to my inexperience, I could not continue after this. The girls would be like "What?" or "okay?"and i would just walk away.
how can i continue the conversation?

I think I'd never use it lol. Find a new opener that works for you.

1. Experimentation is good. Don't let anybody tell you otherwise.

2. Read up on 'negs' again. It seems you don't understand its purpose and/or the correct application.

3. This particular opener is too confrontational and doesn't allow much room for a 'release valve'. You've driven her into a corner . . . this is why you're having trouble finding a way to continue. If you want to reach into their 'drama side' of things, push it even further and get 'absurd'. Making up one now:

"Wait a second. . . I know you. Don't move, I'm calling the cops. (Pretend a call) Oh, I read about you . . . I still have no idea how you escaped from the rehab clinic but ______ (whatever f'd up celebrity you have in mind) you're going right back in for your own good Missy."

Eh. . . some guys might not like ^this. Just made it up but the idea (if you're going somewhat confrontational) is to allow room for nervous/negative energy to flip towards humor/positive energy. Too many ways to play the game and such little time! Don't be satisfied with 1 opener. Write up 10 more. Do those 10, 10 times . . . have fun . . .

That's a way to aggressive opener. Even in a conversation I would not use such lines (except for what she's wearing really would not suit her).
A simple trick I personnaly use is the following: When a girl would use the same exact line on me, how would I respond?
If I would react on a playfull yet challenged way it's awesome.
If I would smile about it, it's ok to go.
If I were to be offended (like in your opener), drop the line.

You can't go anywhere with it, because it doesn't go anywhere. Kasabi's right, experimentation is everything.

If you're going to try to fix it, try something more like:

Comical
"I don't know if I like that shirt or not."
"Why not?"
"Not sure. But until I make up my mind, just make sure you wear something different to our date on Thursday."

or

Seductive
"I don't like that shirt."
"Why not?"
"I don't know. It feels like you're hiding behind it or something."

Again, these are just rough skeletons, from your own rough skeleton. Experimentation is where you'll find what fits YOU. But do you see how both of those lead into what's going on? Both of them segue into talking about a future date or her, both of which are topics of interest.

This isn't one I think I would use though. And there's no problem with that. Some aren't for everybody, and some are just experiments. If you find this one isn't working too easily, even with some tweaking, don't spend you time trying to make it work, but rather make a new one.

_________________
Jazzy Jeff: "My love for you is like a river,
Like a summer breeze that makes my soul shiver,
One look from you is more precious than gold,
Let's grab some BBQ and go get busy!"

umm you just come off as a jerk right here...way to be

It's more of an insult than a neg.

Style used: "Nice hair, is it real?" in the game which has worked for me.

You can also use: "That's a nice dress, I've seen a few people wearing it tonight...".

- Get a reaction,
- Ask her opinion on something
- move into DHV (tell a funny/catchy story) to create f to m attraction,
- critique her (if she's a HB8.5 and up) - "beauty is common", "what makes you different from all the other girls in here?",
- create rapport - fluffing, finding common ground, cube routine has worked for me (esc-kino during this)
- close (however you decided you wanted to before approaching).

Easy on the insults, negs are supposed to be more flirty comments than an attack. I.e. you wouldn't tell a target that she looked fat, would you?