How to Add Products fields / lists / and more : like Phone field for customer or Salers list subcategories

Hi,

First of all i'm totally new to drupal, have read some documentations and i'm on the low of the learning curve.
I've tryed your invoice module and i've found very usefull : thanks for the good work here !

Now, i need to add extra fields and informations inside this module to use it and don't know where to start or if i could simply ask here, so sorry if it isn't the good place.

My requests :

On the invoice form (creation) :
there's a field description for product : how to add & use a dropdown list (for exemple) to select and paste from a configured list of products (and how to configure a list of products) to fullfill automatically this invoice ?
Should i use other module or go inside the code : not sure which line to follow...

In the same way :
- I need to add more details on customers part of the form like a phone number (for e.g.)
- On the 'sellers' part, I need to add (and need to sort when displaying invoices entered) the vendor name (e.g. : Having 1 company and X vendors and in need of displaying all/part of invoice for 1 company or/and 1 vendors etc...)

As i said i'm totally new to drupal and any advice on how to start to achieve that kind of work would be very usefull.

Thanx in advance,
Regards