Whats the difference between the Uno and Mega2560? Do most people have the UNO?
The uno and duemillanove are the most common boards for starting out with. The MEGA 2560 has more pins, so you can sense more stuff and control more stuff, it also has more memory for your programs, a bigger EEPROM, and that kind of thing. It also has a bigger price tag. I have never filled up the memory on my duemillanove, and I have used codes 1000+ lines long, so the extra memory will probably not be a concern for a while yet. If you want to be able to control more outputs, you can use shift registers or multiplexers on both the uno and mega. The MEGA has lots of benefits, but so does the uno.
Are there any concepts I should start reading up on? e.g I have no idea what a capacitor, resistor does etc...Someone mentioned learning about OHM's law also???
It always helps to know what you are working with - get to know the basic components like resistors, transistors, capacitors and a few integrated circuits. Knowing a few formulae will stop you breaking as many components as you would without the kn owledge, so ohms law is useful. also learn about resistors and capacitors in series and parallel.
All of the above, and more, are explained in the book 'Basic electronics'. It is a fantastic book, although it can be complicated at points, but it is where I learned most of my stuff!
Most importantly, experiment, have fun and learn lots by making things!
Onions.