Now that you have done the first layer, We need to fill in the edge pieces on the second layer. There is only one algorithm for that, which I'll show you now.
In this picture this red and green edge piece needs to be put in-between the red center and green center. Before we start the algorithm we need to line up the green side to face the green center.
This is the way you will need to line up the green and red edge piece. Now you will need to turn your cube back up so it's white side is facing up.
Start the algorithm from here, keep in mind that the red and green edge is on the other side of the cube so we cannot see it in this image and that the Green face is the F face. The algorithm you will need to do is:
F' D F D L D' L'
This algorithm can be done in two different directions, one with the edge piece coming from the left and one with the edge piece coming from the right.
The algorithm we did was the one where the edge piece is coming from the left, and now this is the algorithm with the edge piece coming from the right:
L D' L' D' F' D F
The way you can tell if its coming from left or right, is when you turn it back up the right way then the edge piece will either be on the right or left of the cube.
Now sometimes you will have a piece in the wrong way like this, the way to get it out is by putting another edge piece in its place by using one of the two algorithms above to move this piece out.
Now you should be able to do all four edge pieces of the second layer, congratulations!