Hi there! I can’t add anything more to the advice about tension, other than what Zina and Abily already said. They both gave great advice. I will just say that for me and for the people I have taught to knit, tension is one of the things that just takes lots of practice, and it will get better over time.
As for your stitch–since you say you lost a stitch in the process, it sounds to me like you might have accidentally knitted two stitches together. This is usually done on purpose to decrease a stitch to shape a knitted piece. When you say it looks like a stitch with 2 strands of yarn, do you mean that it looks like a loop of only one strand of yarn on the needle, but there are two stitches creating it? If that’s the case, then this is probably what happened. In this is true, you have two options. You can either rip back to where it happened, and re-do your knitting. Or (and this is probably what I’d do), if you can live with having one glitchy looking stitch, you can knit until you are two or three stitches from the end of a row, and increase one stitch. There are a lot of ways to do an increase in the videos section, some more invisible than others. If nothing else, you could practice learning a new knitting skill!
The other possibility I can think of is that you dropped a stitch. Examine your knitting closely (make sure to look at both sides). Is there a loop sticking out where it looks like your knitting unravelled? You would see this somewhere down the row from where the glitchy stitch is. If so, there is a way to pick up the stitch and bring it back up, but if you have knit a lot of rows from where the dropped stitch happened, it can be difficult to do because there won’t be much room to weave the stitch back up. The video on how to do this is in the Knitting Tips section of the videos. (It’s called “Fixing a run/dropped stitch”.) If you dropped a stitch, and picking up the stitch doesn’t work, you also could do one of two things. If you aren’t concerned about perfection in your knitting. You could cut off piece of your yarn and unravel some of the plies, or use matching thread and a sewing needle and tack the stitch down to your knitting, and then after that, do an increase as I mentioned before. Or again, you can unravel the knitting to the place where the dropped stitch “landed” and reknit (I probably wouldn’t do that. I’m too lazy. )
Since you are still learning, I wouldn’t worry about perfection in your knitting. Just keep practicing and your knitting will get better and better.