rmedeiros said:
I mean, i'm a starter programmer!
Either way it is easier to edit the existing project than write a new one from the scratch.;-)
First of all you must design a new board, with similar functionality with the old board. If for instance an I2C real time clock is used, make sure that this clock will be connected on Cortex's I2C as well.
What you could then do is start digging the project and find out which peripherals are used. Then having your datasheet as your ally, start writing demo programms for those parts. You will also need a good book on Cortex-M3, together with a book on C (preferably for engineers). Internet references through google search are also useful.
Since you write a peripheral's demo routine, get in the project again and figure out the settings used like peripheral speed (for this you will need the old MCU's datasheet as well). Now comes the hard part, you must assemble the project. You start a new project and replace the hardware related code with yours. It may sound scaring, but until you reach this point you will have learned a lot about your new MCU and it will look a bit easier. Or you could do it the other way, that is setup a project of yours with all necessary peripherals working, and plug the high level routines of the other project into yours.
When time comes that no compiler errors are seen, the debugging part comes into play. You must verify that the project's behaviour is the same and fix all bugs that gets in the way. But first of all, gather (write or find) demo projects of your MCU peripherals and you did a solid start!
About Cortex-M3, here are a few references to start with.
https://www.eecs.umich.edu/~prabal/teaching/eecs373-f11/readings/ARM-Cortex-M3-TRM-v2.1.pdf
Cheers!