Okay, look at the flowers from the farm. Replace the flowers with the tiles you want to place, like the back where the tree is.
Also, the tree is in the back ground so it should not be the same color as the ground. Since the ground is the same it makes the tree look like some word peice of ground sticking out and that it will stop you. Your palette in general needs a bit of work because all the colors are too close together and the should ground isn't very bold. Composition in general needs improvement, but you're new to this so I'll try just a few instructions for now.
On the solid objects make the brought colors brighter and the dark colors darker, this makes them seem closer. You can also make them different shades of color. Pixel usually made his darkest color a really dark blue even when the tile set was red or some other color. A decent color theory rule is that; red compliments green, blue compliments orange, and yellow compliments purple. If an object is green the shadow is slightly red and if it is yellow the shadow is purple. So the colors in between fade into it. When I get a chance I'll make some image to show it off.
Try looking at pixel art from other games, maybe ones that are stylized. Yoshi's Island or Sonic the Hedgehog have lots of style so you can look there.