I just had what I think was a low level lucid dream last night. I found myself trying to convince my friends that I was dreaming and I decided that I would do so by jumping to a new place through a mirror. I looked away and invisioned the place I wanted to go in the mirror, but when I jumped at the mirror nothing happened. Later on I also found myself trying to make a sign appear on a wall with no success.
Is this just a low level lucid dream in which I don’t have too much control or is this due to doubt?
If you are absolutely sure that you are, indeed, dreaming, then it’s just a lack of confidence and experience.
I sometimes have trouble with this, too. The key is to really will something. Use your will power. I don’t know if you’ve read LaBerge’s “Exploring the World of Lucid Dreaming” but there are ways to increase said ability. It’s important in dreams, because when you’re lucid, whatever you expect to happen, happens.
If you want to jump through a mirror to a different place, but doubts still remain in your mind, then those doubts will, more often than not, ruin your plans.
Basically, in a lucid dream, the amount of control you have is directly related to your awareness of being in a dream. Consequently, what you can do IN a dream is also related to your own awareness of your “powers”, so to speak. I wouldn’t worry too much. Confidence comes with experience, but really, you should try to enforce, in your mind, that you can do anything whilst lucid. In time, it will happen. It’s a mental process. You can’t just say “Oh, I’m dreaming. I can do anything.” You need to REALLY convince yourself. You have to know it. Feel it.
Well, for starters, congratulations for your lucid! A LD is always a satisfactory experience. 
Secondly, most of the beginners think their time in a LD is limited, and tend to rush things in order to get the most out of it, but that only works against their desire to stay lucid.
Instead, next time you get lucid, you must stop what you are doing, Friends, flying, dinosaurs can all wait: the first thing you must do is always get more lucidity. This either by spinning, rubbing your hands (some find jumping jacks useful), feeling textures of the dream world around you, screaming “Lucidity now!” or something related. Take time for yourself, and savor that feeling of lucidity, it is very important to get to know it in order to get lucid more often.
You must convince yourself, at that point, that what you are experiencing is YOUR dream, and thus you are in complete control. Feel the power through you, and know that you will succeed.
When you try something, it also helps a lot to visualize it (ex. when teleporting/stepping through a mirror, it’s best to imagine the place you wanna arrive into first), and the more senses you percieve something with, the better.
Hope i helped, and good luck for your future LD’s! 
Thanks for all the feedback! I actually have read Exploring the World of Lucid Dreaming, it’s a great book. I’d like to try some of the techniques suggested in there. However, all 3 of my LDs have been DILD. I would work on prolonging lucidity, but just the occasion of a LD gets me too excited to focus on those important things. I try MILD pretty much every week night, waking up 3 times a night just to practice. I often find though that when I get up to turn off my alarm I forget to keep in mind my intention to LD. I’m hoping it should all just come with practice 
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I can assure you, it will come with practice. After about 20 LDs you should be having no doubts whatsoever about your abilities 