So, I finished C25K. That makes me an expert, right? Haha, no. But I ran for nine weeks, so indulge me.
The first few weeks of C25K have intervals which are hard to keep track of manually. You can use a stopwatch and write the intervals on your arm, but that's still confusing, and you're going to fall into the ditch that way. I used C25K Pro on my Android phone carried in an armband. This app has configurable voice cues, which are are nice for running without constantly craning your neck to look at your upper arm. There are similar apps for other smartphones.
Or get a fancy GPS watch. The Garmin Forerunner 305 is right around $115 on eBay right now, and that's an amazing deal. You can program in all the intervals, as well as easily pull your GPS track for later inspection/obsession.
Find some way to keep yourself on track. Go running with a partner. Keep a log of each run. Tell people you're doing this. For me, even if I didn't really want to run, these outside pressures got me out there, and I never regretted it, even those times I thought I was going to die on the trail.
Get some gear
The $35 athletic shoes you bought at Walmart are probably not going to cut it unless you're not fat and/or want to run really slowly. I was amazed at the difference a good running shoe makes. Ask an actual runner to help you figure out what to get, and if you shop at the right time, you can find a pair of last year's model at a good discount. I ran in Brooks Ghost 3s, which were under $60 because the Ghost 4s ($100) had just come out.
Get socks. Don't get cotton socks. Cotton socks will make your feet sweat and give you blisters in creative places.
Wear a hat or something. Unless you have super-absorbent eyebrows or don't sweat like a fat man, you need something to soak up those precious bodily excretions.
I'll write more about gear later, but this is the important stuff.
Each day of C25K includes a 5-minute warm-up walk. Make this a brisk walk, and that's all the warm-up you'll need.
However, you should do some serious stretching after your run. I spend at least 10 minutes doing various yoga-style stretches right when I get back from my run, and this prevents me from having to walk around like a zombie. If you're still sore after stretching, it might be time for icing and/or drugs (ibuprofen or naproxen).
Water, that is. You'll feel horrible if you go running when you're even slightly dehydrated. It's hard to make up for being dehydrated all day by chugging a glass of water right before your run (and it'll just make you have to pee right as you start anyway). So keep sucking down the water all day before you run. It's a game! The drink-until-my-pee-is-clear game!
Pay attention to your dumb body
Sore is fine, but shooting pain (among other things) is not. When you start, you'll be amazed that so many of your body parts have the capability to be sore. Stretching helps with this, but know when to take a day off (or another day off). If you have a really bad run, repeat it. If you have to take a few days off and your C25K "week" gets screwed up, don't worry about it. Just keep working your way through the days, and you'll get there.
I noticed that I'd chase weak points around my body. When I started, the intervals were short enough that my legs didn't really notice it, but my lungs constantly felt like they were going to explode. Once my cardio strength got a little better, my shins objected. A couple of weeks stretching and icing after my runs, and then my hamstrings decided to say hello. So don't get too discouraged by what's sore or bothering you this week. Something else will surprise you next week.
Embrace looking stupid
You will look dumb running. People will wonder what's wrong with you. Small children will point and laugh.
Leave your dignity and pride at home and just do it. Just think about all those fat people you know who aren't out here getting in shape. They are going to be so jealous of you, you sexy bitch.
This mentality also goes for choosing what to wear and what gear to buy. When possible, get the ugliest color clothes and the most uncool gear possible. Wearing these will force you to not take yourself so seriously. You can't possibly give a crap what you look like when you look like an asshole anyway.