OK, home from work so I can get to my pictures.
I suggest you think of things this way. You will need a current sensor to sense (measure) the AC mains current. There are several you can use with the CT (Current Transformer) among the more popular for an application like this. Attached are a few CT images to give you an idea of what a CT is.
The current carrying conductor forms the primary of the transformer and is passed through the center of a round iron core, the windings seen in the second image form the transformer secondary and provide a current output proportional to the primary current.
Now I strongly suggest you do a Google of Current Transformer Basics and read in detail
links like this one to gain a good understanding of what they are and how they work. I suggest you read about and understand Burden Resistance as it is very important when using a CT.
You can buy one relatively inexpensive or as BrownOut suggest you can build your own. Building your own for a one time inexpensive project is a great way to learn.
There are other current sensing devices out there. I have included a picture of one such device which is a Hall Effect Current Sensor. These devices provide all the needed signal conditioning. While not cheap they do a very nice and accurate job.
A unit like this one runs about $100 USD. The beauty is for a given current range they output a linear voltage of for example 0 to 5 volts making them ideal for analog inputs of micro-controllers and other analog to digital devices.
That should give you a start.
Ron