1--there are lots of ways to cast on, and one doesn't work, try another.
2--there are lots of directions, and one set doesn't work, try another.
(we all learn differently--and not all video's are created equal!
there is nothing wrong with you.. its the instructions that are at fault!)
1--you can do a simple cast on.. (a simple loop of yarn on the the needle
this can create a loose messy edge.. but if its going into a seam, not matter! and the edge is very stretchy (which can be a good thing (in a sleeve) or a bad thing (a purse/pocketbook strap/handle!)
2--you can do a Knit cast on, (or a cable cast on)
knit is stretchier than cable.. (so both work but are you knitting a sleeve? or a handle(or something you don't want to stretch out of shape?)
3--trickier (and not for you at this point) is a crochet cast on (and yes, you can start it in the middle of a row!)
4--others cast ons
OK so what you need to do is look at a few cast ons'
(if you don't like/can't get the simple loop cast on, then try a knit one!)
here is a link to my YOUTUBE video of a simple cast on.
if this doesn't help, try looking for others.. eventually you'll find the right teacher!