For just starting out, iMovie 09 will work great. When you are ready, you can also step up to Final Cut Express, which is about $150 through Amazon or $200 through Apple. However, Final Cut Express, also uses AIC to transform your AVCHD footage.
AIC = Apple Intermediate Codec. This does a great job of converting your AVCHD footage (the format used by your new camera). However, there will be a little loss of quality that you may not even notice.
The best options for editing AVCHD on a Mac are (in my opinion)
CS4 - Production Pro - this will edit your AVCHD footage natively without converting to a new format for editing. However, you will need a Mac Pro, plenty of RAM, and a great video card to do this. This software runs about $900
Final Cut Pro - Converts the AVCHD footage to Apple's Pro Res format, which is a "virtual" lossless format. This makes the footage much easier to edit with, but take a lot of hard drive space.
To import footage from your camera, you need to (1) open iMovie 09, (2) connect your camera with a usb cable, (3) turn your camcorder to playback mode, (4) follow any prompts on your camcorder, (5) iMovie 09 will read the camera memory or card inserted in the camera and prompt you for with clips to import and/or archive, (6) after import, you should be ready to edit.
Hopefully, that helps. I now use Final Cut Pro, but started with iMovie, then Final Cut Express, and eventually Final Cut Pro. Have fun with your new camera!