You have your needles and yarn, and you've made a slipknot. Here's how to get the first stitches onto the knitting needles.
Let the yarn slip off your thumb, and pull gently on the yarn to tighten the new stitch on the needle. Repeat steps B through D until you have the number of stitches you need to cast on, as indicated by your project instructions. Now you're ready to continue with one of the two basic stitches, the knit or purl stitch.