Generally, I thought it was okay but not great. You had a few mistakes but it was pretty okay. I found the beginning and a lot of the rest of it plain. Especially in the beginning which had several screens of flat. Some detail, in general, would be nice.
Two screens to the left of the area where you get the yellow key has two small floating square platforms. If you fall down and to the right of that, you get a void and wallswim. Also, if you choose to take the long way with the double jump and high jump powerups, the whole level sort of falls apart. There are voids in several places. (That is, with those power ups go through the level again.) Also, the grass in the purple key room was mis-assembled. In lots of places, I couldn't tell what was solid or not, so that could be fixed as well.
But don't get me wrong; for a first try it was well done. I did have fun, and you have potential. Keep experimenting.