Make a plan. Do you aim to get healthier? Run a mile? or two? Set a goal that not's weight related but that improves your overall wellbeing.
I focused too much on the weight and it's really hard for me to lose those 3 lbs I gained because I'm intensely focused on the scale. Now I'm changing my goal to running 2 miles without breaks and not bingeing when I get home from work. Seems better than "MUST lost 2 lbs this week!"
I'd tell a new person to lose the "all or nothing" mentality. I carried that around for years, and it kept me on the wrong track. I thought if I ate a particular food or went a day without exercising, I'd "ruined everything" and might as well give up. This is why I can't do any diet that forbids certain foods -- I have to count overall calories or WW points. If I tell myself anything is forbidden, I fall back into the "all or nothing" trap, where I'm either on a certain diet or I'm off it and I'm eating whatever I want.
Like everyone else said, those small changes add up, and when you see your progress, you'll want to keep at it. It takes a daily commitment and an awareness that you are NOT perfect, but that if you keep working at it, you WILL lose the weight and get healthy and meet your goals.
Give yourself time. You can't expect to lose it miraculously fast and the best course of action is to give yourself at least a year. That way, your "diet" will be a lifestyle change and you will be better equipped to keep the weight off in the long run.
I would tell them that it gets easier the more you do it. The first 2 - 3 weeks are the hardest. Just get past that anyway that you can. After 2 -3 weeks, you will start seeing results and that propels you to keep on going. After 2 -3 weeks the cravings will slow down drastically. After 2 -3 weeks it will almost become second nature to you. After 2 -3 weeks you will be obsessed.
I would tell them to treat themselves lovingly, most especially when something difficult happens.. as we're not perfect all the time and can be quite hard on ourselves if we gain a pound, don't lose a pound, miss a workout, have food that's unhealthy or counterproductive, etc... I would tell them to be kind to themselves and treat all experiences as a useful part of a learning process that they can learn from because they deserve to feel good about themselves and love themselves.
Find non-food rewards!! That has killed me in the past. "If I go for a run, I can have a cookie." "If I lose 20 lbs, I can have a cookie."
Now it's "When I lose 20 lbs, I'll treat myself to a manicure/buy a new CD/get my hair highlighted/etc."
I agree with thinking longterm, but you can't think too long term either. This may only apply if you're a mere babe though. I'm 20 years old, and if I think about doing this for any more than 5 years, I get overwhelmed and depressed.