Before asking a girl out on a date, there are a few things you need to take time to gauge.
Firstly, is she interested in you at all? There will be little clues that indicate whether or not she is, in her body language and also in her verbal language. How does she communicate with you? Is she flirty? Does she hold eye contact with you?
If you think there's a chance she's interested in you, the hard part is out of the way. Now all you have to do is ask. The best way to do this is in confidence, don't ask her friend to ask her, just own it.
If you don't feel confident enough to take the plunge, try asking for her number first and getting to know her over text/calls. This will give you a better chance of familiarising yourself with her and thus her asking her to meet with you.
Getting to know her first is a good idea, because it will also help you with two further problems:
- Deciding where to go and what to do on a date
- Knowing what to talk about when you're on your first date