Why can't you start a new row? The end stitch is often loose, especially the first row, but it gets better after you knit some more. Just knit the last stitch, turn and start a new row. After a few more you'll see it's not as loose.
Or do you mean your yarn has stretched out between the cast on sts and you have some extra at the end? That happens when you do a backward loop CO, so try the knit or cable cast ons instead.
The best thing a new knitter can do is just keep knitting. If they rip out every time you think it doesn't look right, they get frustrated faster. The only way to improve is to practice and so just keep going.
Thanks for posting this question! I am just starting to learn knitting too, and I came on here looking for answers to that exact problem. The last stitches of my rows are too big. So practice makes perfect? Thanks!