First, you should clarify whether you want to have sex with her, or you want to have a more intimate emotional relationship with her. If you’d like to take your relationship to the next level, let her know what you’re thinking about, and see how she reacts.
If having sex is part of that relationship, show her that you’re someone who can be trusted and that you’re responsible; have protection and know how to use it properly.
If all these things are in order, let her know you’re interested in taking your relationship to the next level (and mean it!) at a time and in a way where she doesn’t feel pressured, and see what she says.