First decide on a small project, use the asm template files that is included in MPLAB to set up your workspace. First you should try to toggle some LED's on/off at different intervals, then add a switch to toggle them. After that you can start adding things like ADC conversions etc.
In the beginning I found that searching for .asm files on the web to see more or less how things are done by aothers helped allot. The Microchip website also has allot of source files/templates and tutorials that you can try.