Global connection string under C# with MDI container and Toolstrip with stander items

First time for me to programming and need some help to implement my first project.

However my questions are,

1# How to make a global connection string for my MDI form and child forms?

1.1# Is it right for every time to save or retrieve data from mydatabase use connection string or just use SQL scripts?

2# I created MDIform and it was MDI container, and added toolstrip with some buttons ( Save, Add, Search .. etc) and make this buttons disabled by default and need

A# When open any child form active some buttons as i need? B# When click at Save buttons update my table, but it's one buttons with for many forms so how to detect active form and how to design this function?