Text app and saving files

Hi i have some problems with writing this. Can anybody please help me with this. TT I tring to create a form application in C # .NET. There should be a button to open the file in the form, which opens a file selection dialog when you click on it. When the file is selected (only after pressing the OK button), it should paste all lines in the file that contain the number into the listbox. One such row = 1 item in listBox. Next, it should create a Save button that offers a file selection where all lines from the listbox will be saved to separate lines. 

Thanks in adv for your help ;)