There are many reasons as to why your girlfriend may be hesitant to kiss you. If she has never kissed anyone else before, she may be nervous, which is perfectly normal. Most people who have never kissed anyone may worry that they will do something wrong or that their partner may not enjoy the kiss. You may want to try easing her fears; let her know you understand her fears and will still love her regardless.
The best way to get her to kiss you is to go in for it. Try not to talk about beforehand. Instead, look into her eyes, and slowly move your mouth towards hers. Start off gently and slowly; going to rough and too fast will probably make her back away. Also, make sure your breath smells good; nothing will send her running for their hills more than stinky breath! Good luck!