yeah, just moved from a PC last week, I was just thinking about creating a signiture for my emails but discovered that Mail doesn't have that feature, or unless I can't find it.
Mail/Preferences/Signatures
Chose one of your accounts and then click on the + button located below the middle column. In the right column you put in your text for a signature.
Each account can have it's own or even more than one signature file.
When you create a new message you can choose which signature to use from a drop-down menu on the right side of the message below the Subject line.
For more information about formatting signatures -
Google: leopard mail signatures