Like Grumpy_Mike said. I would add motors with encoders (to measure motor rotations and thus distance) and for starters maybe a rangefinder (like the HC-SR04 or Ping), switches for bumpers and a servo to make a scanner with the rangefinder. Learning those and incorporating them together will get you started. There a tons of kits with the chassis and motors available. I started with an old Roomba that I got cheap at a yard sale. A big decision is what type of drive system to use. There are 2 wheel drive, four wheel drive, tracks, differential steering. Ackerman steering.