How do I resize a video layer (not the compossition) in After Effects? I created a footage of 1920x1080p, then I added a video of 1280x720p in a layer, now I would like to resize the video of this layer to fit the size of my compossition. Is this possible to do without using an external program to reconvert the original video at the desired size?.

Please, take into account in your explanations that I'm very novice using AE, I don't have more than 2 days using it.
Assuming you meant the composition is 1920x1080, and you wish to resize a layer to match the composition size, do one of the following: 

Right-click on the layer name. In the menu that comes up, click on Transform --> Fit to comp.

Another method is, in the controls for Scale, under the Transform drilldown, right-click on one of the values. Click on Edit Value.  Change units to % of composition. Alter the value of the larger dimension, out of Width and Height, to 100%.