F2L (First Two Layers)
After cross comes the F2L (First Two Layers), arguably the most important step of Fridrich Method. This step completes the first two layers by placing the four corner-edge pairs one at a time into the four slots between the cross edges.
Rather than blindly applying the algorithms, for an efficient F2L it is important to understand how each one works. Beginners should first read Getting Fast with an Easy Method for simple techniques in a simple Layer-by-Layer method. Learning F2L teaches mostly intuitive F2L, breaking the pair patterns into groups with similar solutions. Then Standard Extensions to F2L teaches some basic F2L techniques requiring no extra algorithm. To really master F2L, work on these techniques while learning intuitive F2L.
A Note on Cross Placement
This page lists the F2L "algorithms" with the cross on bottom. Most top speedcubers place the cross on D or L during F2L (more rarely R). Although more cubers use D-cross because this is how F2L was popularized, both placements should be equally good. Personally, I mostly solve with D-cross but sometimes switch to L-cross (or rather tilt in the z-direction) depending on the case. Avoid placing the cross on top. This gives you very little view of the last layer, which contains the pieces you want to move.