1. It is important that you plan the whole project well in advance with set milestones. If you don't plan, you will find it difficult to finish the project. There will be programming problems, there may be delays due to the fact that a part you were planning on using died, or your prototype does not behave as intended.
2. If you get stuck, ask questions at piazza.
3. It's preferable to divide the project in small, manageable tasks, so that you continue to gain confidence as you progress through the project.
From these stores, you may buy mechanical or electronics parts, such as sensors, batteries, in particular anything that I did not supply to you in the kit
Maker SHED
MAKE Magazine’s store can be found online at http://www.makershed.com/ and in real life at Maker Faire (http://makerfaire.com/).
Adafruit Industries
SparkFun Electronics
Electronix Express (your kit is sourced from these guys)
Polulu Robotics (an attractive site run by young kids with a passion for robotics)
Mouser Electronics
Digi-Key Corporation
Newark
Mouser, Digi-Key, and Newark are all well-known stores for buying electronics components
All Electronics Corporation (good for hardware, like gears, chains, sprockets)
Ebay
McMaster-Carr (a very reputed store for buying both mechanical and electronic parts)
You could also buy parts for building robot structure at Home depot or Lowes.
Vex Robotics (they seem to have a good selection of gears, sprockets, tracks, for building robots)