Hi Miguel,
The best timing to talk to her and ask her out is when she isn’t busy with customers.
Approach the barista at this time and keep it very simple.
It doesn’t matter if there are coworkers around. They will mind their own business.
Avoid trying to talk her up before asking her out. Keep the conversation brief. Exchange a few pleasantries and get right to it. If she likes you, she will give you her number.