Training for Sprint Triathlon

Congrats on getting "roped in" to triathlon.  

Every person who signs up for a race does so for unique reasons and with very personal goals.  You need to identify what those are for you and the rest will follow.  

I did my first triathlon in 2005 and went from "maybe I should do a tri" to finishing in about 3 weeks.  I'd lost weight and was exercising daily for a backpacking trip and when I got back from the trip, looked for something else to do and that's when I found triathlon.  (I knew about the sport from TV coverage and had lifeguarded for a race 20+ years prior.)  I showed up having swam twice (I was a lifeguard and swam for fitness over the years, but hadn't swam recently) and rode my bike about 3 times with any real purpose (I'd commuted on a 10-speed years before).  I don't recommend this strategy, but it worked for me because I already had familiarity with all 3 disciplines even if I hadn't been doing them a lot.  

Plans:  Beginner training plans for sprint triathlons may be 8-20 weeks long and rarely have you doing all three disciplines in one day.  If your race finishes with a 10K run, it's probably the longer, "Olympic" or "International" distance triathlon (1500m swim, 40 km bike, 10 km run).  You might be looking at a 3-hour race (give or take) for this distance where a sprint may be 1-2 hours.  A plan provides some guidance and structure, but most of the "canned" plans like you can download here make assumptions that may not be appropriate for your specific situation.  They balance workout efforts with rest and recovery to minimize the potential for injury.  The closer you can follow a plan, the more consistently you will progress towards your race.  One key to training is to recognize that a quality workout is more important than thrashing around believing you must do it all.  The plan may call for an easy 30 minute run and you feel like you'd be better running hard for 45 minutes.  And...maybe you do the hard run instead only to get on the bike the next day and not be able to execute THAT workout because your legs are shot from the day before.  

Race experience:  If you want to run a 10K, sign up for one (probably at least a month before your triathlon).  If you can't find a good race or aren't really motivated to run a 10K, you won't miss that experience for your race.  Triathlon runs aren't like stand-alone running races of the same distance unless you're at the pointy end of the field (i.e. you might be able to finish first overall).  For the rest of us, they're often worlds apart.  

Swimming:  Rather than show up at a swimming pool and thrashing around, consider getting some lessons (group or private) because there is a lot of technique involved and you'll advance faster if you build good habits from the start.  

Finally, look around for a local club, and if you find one, hang out with them.  You may get opportunities to swim, bike, or run with other people and they'll be able to answer many of your questions (plus, they're usually a lot of fun to hang around with even if you aren't training).