Search a sentence in a string

[C# Programming]

Hi,
My question has two parts:

1) How can I search for a sentence (e.g., Dell Canada) in a string (e.g., I am working in Dell Canada, and I found it...) .

2)The second part is my string is text in a RichTextBox, so I would like to find the TextRange of that selected sentence and apply certain decoration.

thanks.