Stockinette Stitch

When I start knitting, I have 10 stitches instead of 9. I started casting on with 9 stitches including the slip knot. I’m not sure how I came up with 10. Please help. I made this mistake more than once.
Can someone please show me how to make a slipknot properly.
I think that’s where the problem lies. Also the last stitch I knit is always loose.

Maybe you counted 10 sts after you put the first loop/slipknot on the needle. That first one counts as casting on a stitch too. You don’t have to begin with a slipknot though, just loop the yarn over the needle.