Has she been got at with another dog while she was in season, or is this something you planned?
The only sure way to tell if she is pregnant will be to have a scan at around 4 - 5 weeks, and she will be 'pregnant' or in whelp for around 59 - 63 days.
As this is obviously your first litter you need to get some advice as it is not as straight forward as some people think.
Your bitch will also require extra food from around 4 weeks, usually puppy food and some protein (eggs etc), which is why you really need to be sure if she is in whelp or not.