Tips for a new comer?

I am relatively new to triathlon training, and have not had the guts to enter an event yet. The main reason I started the training is because I enjoyed the variety, and it is cost efficient (only have to pay to swim) and you dont have to be stuck inside a gym all the time. I am also desperate to lose weight!
I have a couple of questions for some more experienced athletes, and particularly those who started in a similar position to me. I am 25 years old, slightly under 6ft tall. This time last year I weighed 13 stone (which I consider to be my "natural" weight), now I weigh 15 stone. I am desperate to get back down to my old weight. What kind of effect would losing 2 stone have on my swim time and run time? I currently swim 1500m in 26mins, and run 10k in around 45mins.
What is the best way to lose this weight? I currently run (about 7k), swim (1500m), and cycle (20-30k) twice each per week.
Also, how can I replicate road riding on a excercise bike (since I have one of these but cant afford to buy a road bike at the moment)?