Simone,

1. Define the problem first.

2. Determine the resources needed, such as RAM,EEProm,program memory,external peripherals,etc.

3. Write a flowchart of the tasks that need to be done.

4. Then write the software.

5. If you have written in C, then you must take some time to learn the nuances of C for embedded processors, there are many functions that you will have to write on your own or borrow from this forum.
I myself prefer assembly language, but have written programs using Keil C. The main complaint I have is that you have to be extremely careful when using C, that you do not use up the stack, by nesting a lot of subroutines. But this issue is becoming less of a factor with processors that come with 2-3K of RAM. You have to conserve resources when you only have 255 bytes of RAM. You definitely need to learn all about the processor that you choose, so that you can program effectively.

Jay