There are many resources to help you get started. You can try my Tri Newbies link on the sidebar. There's a ton of information out there. However, here are a few suggestions.
To get started with training:
1. Take it slow.
If you've been a couch potato, don't overdo it. Your body isn't used to it. 30 minutes of activity 4-5x a week is fine for the first few weeks. You'll know when it's time to increase when it feels easy.
2. Set your goal.
Training is best if you write it down. Write down your goals and map out how you are going to do it. If your goal is to finish a sprint triathlon, plan on giving yourself 8 weeks or more if you've never done one. (A sprint is the shortest distance offered in triathlon, usually consisting of a short pool swim 300m-500m (6-10 laps in a 25m pool; 1 lap is down and back), a short bike (10-17 miles), and a short run (2-4 miles).
3. Build a plan.
Now, figure out how much time a week you have for training. You will want to build up to doing each sport 2x a week to maintain fitness and build endurance. You should not need to do more than 1 workout a day, although you may find it easier to do some days with 1 workout in the morning and 1 at night. Make sure you give yourself days off! Your body gets stronger when you rest and allow your muscles to recover.
Plan the types of workouts you will do. If you have 2 swims, 2 runs, and 2 bikes a week, figure out how you will spend your time in the pool, on the bike, or running. I usually like to mix it up. Follow hard training days with easier recovery days. For instance, if I do a distance swim and a hard run one day, I might do an easy bike the next. That way, I'm not overtaxing my body and I'm challenging it in different ways. This allows for faster recovery. There are a million types of running, swimming, and biking workouts you can do. Keep it simple to start with, and go from there.
4. Sign up for your race.
This will commit you and motivate you to stick to your plan. There's no excuses after you sign up for a race. Look on active.com or local websites. I have listed a good link above to help you find local races in your state. There are many sources on the web to help you find local races. You can usually sign up for them on-line.
Get outside and start training! Don't forget to have fun!