How to add new fields to opportunity form view ?

I'm newbie in OpenERP developpement, and i want to add some fields to opprotunity form view. 1) I want to add Picklist field for Stage and make probability field ReadOnly. Probalility should depends on stage change. 2) I want to add new field of type float (coefficient), ReadOnly and it depends on opportunity's probalility on the timing of creation: If probability <50% then coefficient == 1 Else if probability >50% then coefficient == 0.1 Else coefficient == 0.5 Once the coefficient is calculated for the first time, it should not be changed.