The answer is going to be pretty much the same for all three sports, in that you need to do a fitness/threshold test, and then use that to compute training paces.
For swimming, I recommend the 3x300. Warm up completely, then you swim 3 different 300 meter swims, with :30 seconds rest in between. You want to try to keep them as close in time as possible, while still pushing it hard. Something like 3:30, 5:00 and 6:45 would not be a good test. Once you have your times, then determine your average pace/100m and use that to base training cycles. There is a series of books called Swim Training in a Binder that have workouts, and they outline a fit test in the beginning section. There is also one that is triathlete specific.
For cycling and running, you'll need a HR monitor of some sort. Do at least 15 mins of warmup, then it's basically a 30 minute time trial. At 10 minutes into the test, hit your HR lap button, so that you get your average heart rate over the last 20 minutes. Do a good cooldown after the test. As I don't personally like HR based training, I also try to gauge my perceived exertion (RPE), and use that for later training. There are, however, many people that like HR based zone training and many plans are based off of this as well.
Since you are fairly new, and planning a triathlon workout regimen can get a bit involved, I would recommend a book that has a plan, or a site such as Beginner Triathlete that has a lot of premade plans, some free and some paid, and an EXCELLENT support and discussion community, centered around beginners.
Now, as a part time coach, I know that for a lot of beginners, the big hurdle is swimming. If you aren't a former swimmer or at all uncomfortable in the water, I highly recommend getting as much pool time as you can, and get in some open waters swims (OWS). OWS is a very different animal, and even experienced pool swimmers can freak out the first time or two. Also, if you plan on using a wetsuit, get some practice time in OW in the wetsuit. It's also a different beast. Don't OWS alone, however. Go with a group or a friend or two.
I would also recommend signing up for a sprint or two before you do the Olympic distance. You get used to the environment, transitions, what it actually feels like going from the swim to the bike to the run, and you get an idea of where fitness/training is lacking.
Edits for clarity: You will periodically want to redo all the fitness tests, since the numbers will change as you get fitter. Also, do not attempt to all the fitness tests on the same day, or even back to back. You might be able to get away with a swim test on one day and a second test the next day as there is no impact with swimming, but I would space them with at least an off day between so that you get the best results.