first ... WELCOME !!!! it is good to have you here !!
like the others have already said, you have ALREADY taken that FIRST step to a healthier thinner you !!
start small... that is the easiest and best way if you jump in head first and full steam ahead (gee that sound familar... oh yeah i just posted that sumwhere else

) its the easiest way to fall... and we have ALL been there and done that...
i'll tell u what I have done with my soda intake, i have gone from drinking NOTHING but soda all day everyday to 2 a day, an out ounce can with my lunch and about a half a glass with dinner, and tomorow i am going to start curring out that lunch time can...
if you go to the support forum, there as a thread for 21 day challenges... i am new here and just signed up myself a few days ago and i am a HUGE advocate for this... i started my first challange 5 days ago, i vowed to get in 60oz of water a day... hey thats only 3 - 20oz bottles... easy !! today is day 5 successfully completed !! and the 21 day challenge can be ANYTHING you want to try to do for 21 days, there are 3 levels you assign the level you think best fits with your goal, the levels allow you pause days the pause days are days you can use to take a break or if you totaly mess you u can use that so it dont count against you, and i think its a great way to get started implimenting new changes into your life, like giving up your sodas...
as for exercise... do you have a game system?? x-box? PS2? game Cube? DDR (Dance Dance Revolution) is a great cardio work out and in game mode there are different levels so you can start out slow and work your way up and most of all the 3 rounds you play for game level are only like 15 minutes, so that could be a great way to start.. i have DDR and havnt done it in a long time but am hoping to break it back out tomorow...
your treadmill idea is a great one but that can get pricey for a really good one ( i wish i could afford and had a place to put a treadmil)
also you can get free weights at places like walmart or target in the fitness and sports section... not sure about targets prices but i know at walmart ( i get everything there lol) the yare like a dollar a pound, so one pound weights are 1 dollar, 3 pound weights are 3 dollars and on up... you could get a set of weights and just do simple excercise with them for your arms and upper body
ummm... what else... do you have a curves around your neck of the woods?? there have been a bunch of girls on here who have great results with that (i'm going to join in a few months) or a rec center or some place like that where you could use the work out room
anyway just kind of rambling and throwing out some ideas
but really you are already doing a great job by taking that first step and there are 4 things to remember:
start small
remember we have all been there
if you need us we are always here
and YOU CAN DO THIS !!!