I'm asexual and it doesn't mean you can't form a strong relationship! To realise you're ase takes a degree of self-awareness, and hopefully she won't be an emotional idiot. Thus, the best advice is to talk to her.
Ase people are often a bit scared of the situation going out of our control - I like to know that everyone knows my boundaries and is very aware of them! Tell her that you think she's really amazing and you'd like to spend more time with her, and that you're aware she's asexual - what does this mean for her? If she's not sure (I didn't for a long time) tell her you'll follow her lead, even if it is 'we'll be friends, spend a lot of time together and not think about this again for a couple of months' - I have to be good friends with someone before I can consider being a partner, but then I'm a terrible romantic! Just make sure you both communicate a lot! It's nerve wracking to allow someone to know exactly how you feel and what you want, but it's a really good idea to do that to help make a relationship work.
Good luck!