Selecting Lines

Hi,

it have written a notepad application in C# 2008 which i am using to open documents with a lot of lines for example 5000+ lines. the problem that i have is that i have created a button that i want to use to select a specific number of lines. For example if i have a document that contains 5000 lines, i want to be able to select 3000 when i press the button. Then i can cut or copy the highlighted lines. This is the code i use to count the number of lines

Int32 lines = richTextBox1.Lines.Length;
Int32 textLength = richTextBox1.Text.Length;
statusLabel.Text = "Lines: " + lines + " Characters: " + textLength;

Thank you for your help in advance