Im new, how to open up 1 form in either Invoice or Quotes

Question

Hi im new and very noob at this so far so im new to code talk and building it quickly as i can :). im busy with a project in web and the form im busy with is a Invoice/Quote form. i got it right to populate the dropdown boxes and auto generate invoice number down. i got 2 problems.

1: im not sure how to open the same form up in either invoice or quote. because its a 2 forms in one kinda like a Client form that you open up in add or edit.

2: i have this one drop down box called service, below it is a grid. Select your service, click the add buttom and it goes into the grid, so that you are selecting a list of service. well i populate the service dropdown box, but how i get it to the grid?

can anyone help me with this. Thank you so much