How to read excel sheet and use acroos all the functions in a same file?

Hi,

I want to know how to read an excel sheet and use it across all the functions and close it while i quit the FORM?

It's like i want to open the excel sheet once at the start of my application in VB and use it in all the functions and close it when i close the applications. I am working on windows forms visual studio 2008 and excel 2003.