The A spot is 3-4 inches (about a finger long) on the front wall of the vagina. It is also called the anterior fornix or deep spot. First train her A spot with your middle finger. Use plenty of grapeseed oil as lubricant and put your middle finger into her vagina. Go in deep and begin "scooping" her A spot out. You're manually contracting her orgasm muscles so she has no choice but to cum. It will take a while at first because she won't be familiar with the sensations. Persist until you give her an A spot orgasm. When she cums, you will feel her anus involuntarily contract.
Give her 3 more A spot orgasms with your middle finger and then use your penis.
Here's a diagram:
