This is going to be super-long, but I remember wanting to be able to ask this question when I was trying to lose weight. So I'm basically writing down everything I wish I'd known from day one of my weight loss.
I too was 315. I lost 100 lbs down to 215, but since then I have gained about 20 back over 3 years thanks to excessive travel due to work. So I'm in the process of losing it again.
Here's my advice:
1. Go to the gym. Start slow, but work your way up. Eventually I was such a regular that the girl who works as the secretary became one of my best friends.
1a. WEIGHT TRAINING - I know you're looking to lose weight, but one of the major keys to that is raising your muscle mass. Cardio is great for weight loss, but the metabolic benefits of cardio end pretty much the minute your heart rate returns to normal. Weight lifting, even small amounts will help you A LOT.
1b. CARDIO - You need to build up endurance because you're going to want to be getting A LOT of exercise. Start slow (I'd recommend stationary bike or eliptical trainers if they have them, it'll take the stress off your knees) then treadmills, rowing machines, Stairmasters, whatever. So long as you keep that heart rate up and you last longer. Also get some fitness magazines for guys, they're usually instructive, full of hotties for motivation, and good at keeping you from looking at the evil LED display on the equipment.
1c. STAY AWAY FROM MAGIC BULLETS - My gym had all sorts of crazy muscle elixirs, gatorade, supposed fat burners. Wastes of money, and if you don't know what you're doing you can wind up doing damage. A buddy of mine thought he'd have one of the protein bars that advertises to "build muscle", well the bar assumes you don't have any weight on you, so it had weight gainer mixed in.
2. Watch what you eat, drink.
2a. EAT, BUT EAT RIGHT - Big mistake lots of people make is not eating enough. You don't have to starve yourself. Doing so only shuts your metabolism down and makes it harder to lose weight. Eat when you're hungry. Drink lots of water, often we think we're hungry when we're thirsty. Remember that we usually don't feel full until 10-30 minutes after we are. No fried anything, no cheese, fat is okay as long as it's not saturated (think peanut butter, almonds). I would usually make myself 5 baked chicken breasts before the week began so I could just nuke a premade dinner. Add some rice and a veggie and we're stylin'. If that's too monotonous for you I understand. Do what you have to, but that's what worked for me.
2b. Alcohol, empty calories, esp. beer.
2c. Drink water whenever possible, avoid juice it's not healthy if you can spare the calories, the occasional coffee isn't going to kill you, just don't add sugar, and for god's sake no soda. It's bad for your weight loss and your teeth.
3a. "If you didn't write it down it didn't happen" is a saying in my profession. So I wrote everything down. What I ate, when I worked out, how long, how much weight, how many reps, how was the treadmill set up, my weight, body fat percentage, time for a 5k, etc. Was helpful to me.
3b. I put pictures of my family on the refrigerator. My uncle had died of a heart attack (at 37). My step dad has diabetes. and the rest of my family I love too much to not spend as much time with them as I can, and damn it it may be vain, but I want to look as good as I can. I've heard variations on this a friend of mine put hot guys on the fridge because he always felt they were judging him, kept him from opening the door. Another, more frank friend, who had been diagnosed with diabetes put on her fridge a little note: "Do you want to fucking die?" Takes all kinds I guess.
3c. Goals. Set yourself some goals, realistic ones, but don't start setting them until you've got a week under your belt. That way they can be realistic. Maybe it's get to the gym 4 times this week, or maybe it's lose two pounds, hell maybe it's
3d. Don't weigh yourself often. Weight fluctuates with how much we've been drinking, eating, etc. Think of the stock market. If you look at the market from day to day you'll see it goes up and it goes down, but if you look at it over 5 year spans, it's almost always up.
3e. Check your body fat percentage. I got a little machine from costco that's like 10 bucks and takes a watch battery. It's not super reliable, but sometimes you'd look at the scale and say "Shit I gained 2 lbs" and then look at the bf percentage and say "Ah, but I lost .5 percent body fat.
3f. Find ways to reward yourself. Maybe you'll buy a new outfit. Maybe you'll take a vacation after losing 50 lbs. Just do something, even if it's naughty. ;)
3g. Feedback. If you've got a group of friends or coworkers they'll start saying something.
3h. Keep at least one pair of old clothes. I pull out my 48" waist jeans whenever I'm feeling glum.
3i. Pictures. After I lost the weight a friend of mine who had been in India for th