This should do:
string a = @"If I had to live my life without you near me
The days would all be empty
The nights would seem so long
With you I see forever oh so clearly
I might have been in love before
But it never felt this strong";
string[] lines = a.Split(new string[] { Environment.NewLine }, StringSplitOptions.None);
StringBuilder s = new StringBuilder();
for (int i = 0; i < lines.Length; i++)
{
s.AppendFormat("{0}. {1}{2}", i + 1, lines[i], Environment.NewLine);
}
string newString = s.ToString();
Cheers