Validation Of Inbuilt SAP Control

Hi All,

Can anybody suggest me how to give validations in SAP inbuilt forms.

Means In BusinesPartnetMasterData form Tel 1 and Tel 2 textbox control i want to give validation like "Enter only number" or other textbox "Enter only character".

And i also want how to add our own cotrol like teaxtbox or label in that BusinesPartnetMasterData form how to add ?

Can anybody explain me step by step.