I am an amateur programmer, with limited PIC chip experience. I am interested in using the arduino, but do not know what kind to purchase to begin. I want to use it to control lights, interfacing with heat/movement sensors.
Well then, you should probably buy an Arduino, some lights, and some heat and movement sensors...
(Sorry. All this agonizing that people do over buying just the right starter components. Just dig in and go for it! I mean; cheaper than taking a date to dinner and a movie! Even if you get "not quite" the right parts, you'll probably find uses for them, and sometimes you learn more from mistakes than guessing right, anyway.)
As much as I dig westfw's no BS answer, I do understand the need to get the right parts to begin with. I live in a country where electronics are expensive (I paid 47 us$ for my duemilanove), getting things shipped to is slow, risky and expensive, I have limited funds and a parts wishlist that is looong and growing.
So...I advice you to do some research on the options. A standard board has 14 digital I/O's and 6 analog inputs. If that satisfies your needs go for that. Otherwise the MEGA boards have 54 digital I/O's and 12 analog in's (the Seeeduino MEGA board is an inexpensive option here). It depends on HOW MANY lights and sensors you wanna connect.
You may also wanna think about HOW you wish to use it. Some boards are specially made to use with breadboards for instance (eg. iDuino and Boarduino), some are specialized for servos (RoboDuino) etc...
So think it through, do some research and get the board you that fits you...
Yes, you should go with a stater kit. I bought ladyada's as my first introduction to arduino... the kit even comes with a great set of lessons online to get you started. Some other shops sell starter kits as well. There should be a thread that lists them up in the News category i think.
I just got an arduino recently and I would recommend soing what I did, buy the starter pack from adafruit.com. It comes with instructions and really helps to get you started. If not, I would prefer the classic duemelinove board, it is more versatile for lots of different things to do.