1- The main subject here is the canon and its way at the bottom of the picture. There are way too many blurry trees in the top part. Nice exposure on the picture though. Recompose the shot, get closer to the angle, try getting lower down on a day with a nice sky, look for interesting angles that play well with a blurry background
I would also of liked to see the canon barrel in focus. You have the wheel in focus, but having the barrel (which to me is an important part of the canon) in focus is key. Keeping the back wheel out of focus is fine. When you head to some place to take pictures, something I try to do (being new to photography as well) is take at least 10 pictures from various angles, distances, settings and effects. I try and apply this as much as I can when taking any picture and you'd be surprised how this makes you see things differently
2- It’s a rock. Not doing much for me. I do like your depth of field and you managed to capture a crisp rock, with decent texture in it. But the composition is totally off, except for having a foreground out of focus like that. Rule of thirds is a good start.
3- I actually like this one. The effect makes it look like brush strokes. Interesting and nice job experimenting (which is what all us new photographers should do more of)
4- Interesting picture. I find the background in here distracting, even if its out of focus. Nice depth of field though! The picture has a slightly yellow tint to it, check your white balance. I also find that the carpet is distracting. So good concept, fun picture, but look for a more solid and less busy place to put the guitar.