well, as mentionned above u should take it one step at a time. start by knowing how to use ur ports, then move on to more complicated things like delays, then move to timers, then understand different important internal registers. for practise, start by switching an led, then make it blink, interface with an external device like a temperature sensor and a 7 segment display, then move on to a bit more difficult stuff like make ur own watch and display it on an lcd. then u can play around with serial comms, and I2C things. it all sounds cmplicated, but if u can cover it, u can be sure you have a pretty good idea of microcontrollers!
good luck!!!
alex