I was in your same position about a month or two ago but I've grown and learned so much about photography since. I can tell right off the bat that have you a natural talent for good composition. I'll try my best to explain how you could possibly fix some of the shots.
1) Your focus is what people focus on (go figure) when they first see a picture. It should generally be the subject, but in this picture there's nothing interesting too interesting going on in the focus. I think this picture would look great if the couple was in focus. I'm not 100% sure if you know how to use the 9 point auto focus system but it's quite simple. While shooting, if you hit the upper right-most button on the back of the camera you can select which of the 9 red dots in the viewfinder you want to focus on. I won't go anymore specifics just in case you already know what you're doing.
2) Very sharp picture and again good composition. My only complaint would be that the background is very complicated, and to make matters worse, it's all in focus. This picture would look much better if your aperture was more open so that they can really pop away from the complicated background. Quick and dirty way to do this: set the camera to Av mode and select your desired aperture. The lower this value, the more bokeh (blur) there will be on whatever is out of focus.
3) My favorite of the set. The female's foot chopped and male's arm are pretty much the main problem. To fix this just step back. The tracks make great leading lines in the composition.
4) Funny picture, you captured the emotion and humor perfectly. The only problem here is the dark shadows on the face. Two easy ways you can fix this is with 1) a reflector (and someone to hold it) or 2) flash for fill light. I also think if you would have stepped a bit to the right you could have had them between the two trees to frame the shot a little better.
This is a handy and short tutorial to help give you some additional tips:
http://emptyeasel.com/2006/11/24/9-steps-to-creating-better-compositions/
My only other advice would be to read a lot of photography tutorials on the web. I've learned so much about functions on my camera that I otherwise never would have utilized or stumbled upon.
Good luck on your future endeavors and great first set!