Hi selena,
The links provided by dagakshay on his post are great for beginners. And what I could suggest more is to simply select any 8051 microcontroller and learn its datasheet thorougly.
From the datasheet itself you can learn about I/O ports, adc, timers, interrupt, uart etc.
After that try some simple basic codes like led blinking and all.
Also keep in mind that learning about the microcontroller won’t help you much, because microcontroller is used for embedded Systems. So you should learn about other modules like LCD, gps, gprs etc. and the technique used for interfacing them.
Just post your doubts on this forum and you will surely get answers right here itself.
All the best.