How do I tell her I'm in love with her?

Well at least you have the perfect opportunity to bring up the topic after dinner. Just be yourself and dont stress out. Gauge your comfort level on types of questions to ask her leading to the part where you tell her about your feelings. Also be mindful about her answers and her reactions. You dont want to ask her questions that makes her uncomfortable. If she's going out to dinner with you, obviously she likes you. She's already your friend. When you reveal your feelings to her, if she's not with anyone, or if she's in an unhappy relationship then she might be considerate of the feelings and start a relationship with you. The good thing is there's fire in your heart called love. Start a spark on hers and sooner than you know it the both of you will be each other flame.