Educate yourself on things to say to your girlfriend
Women like to talk and want others to listen them. Ask about her, what she likes or dislikes, her childhood memories, her fears, her passion, her embarrassing experiences of life etc. Always be attentive when she talks, and ask random questions every now and then. However, don't be judgmental about anything as it is the last thing the woman will be accepting from you. First try to understand her, and then ask relative questions to support her feelings.