When you are knitting, your yarn is in the back of the work. When you purl, it's in the front. So if you want to, for example, knit 1 stitch, purl 1 stitch, knit 1 stitch, etc, all you need to do before you work a stitch is move the yarn from back to front or front to back.
Once you have that, you can add texture, motifs, and borders to your knitting. Here are just a few useful knitting patterns that combine knit and purl stitches, with written instructions and charts.
K1, P1 Ribbing
K2, P2 Ribbing
Seed Stitch (also known as Irish Moss)
Texture and design
King Charles Brocade