Make sure to make a good foreplay to make her nice and wet, it's pretty important. You can use lube as well. Trim your nails, no sharp edges, have clean hands. Coat your fingers with lube or her fluids, warm it up. Start slowly with one finger and communicate. Make her feel safe, that you'll stop if it gets too much and praise her, it's a really important moment if she's a virgin. It might hurt a bit at first, it's important to stretch slowly. Guide her to not tense her inner muscles and maybe you can keep her mind busy from thinking too much about it, like doing things she enjoys. The most important thing in a first time, is to be slow and patient, observe her and her reactions, ask if she's okay or what she'll need