Both of the previous answers are a great place to start. There is also quite a bit of documentation from Odoo on creating a new module. 

I suggest you to read the code of very core base modules like product, sale, stock.. depending on the area you wanna go deeper the base modules that compose the area will be a good read for you to get a better idea of how everything works and how can be extended

Simple "Partner_Credit_Limit" - its also a useful module, simple. - 

when installing..

• ALWAYS Practice on a test Database (you will crash Odoo)

• Copy to "addons" folder

• If linux change access writes odoo:you & 775

• Update module list

• Re-start Odoo server
  

I'll even set you an easy question - improve the xml  view to put the "credit limit" under "Sale" where it should be :)

Useful links

API

Developer Docs