By the very nature of this node, I can't teach you how to smile here. I can only hope that my sharing my experience can help you teach yourself.
I think back to a time when I was fifteen and full of angst. Realistically, there was nothing wrong with my life, but at the time I had convinced myself that there was nothing right about it either. I won't say I never smiled -- that'd be a lie. But for a while that year, until the night of May 13, 1995, I had lost the ability to smile on my own. There's a big difference between a smile you put on your face and one that just sort of shows up there on its own. The former's phoniness is easily noticed, while the latter's simplicity as a genuine expression of happiness is undeniable when true.
Nowadays, I can't stop smiling. I'm not some sort of goofball that runs around grinning like an idiot, and not some sort of easily-amused simpleton, but sometimes it doesn't take a whole lot.
Just last week I saw a hummingbird land. My first piece of furniture is a tree stump named Clyde. Ben & Jerry's. Pillow fights, bubbles, Batman, hearing San Diego bands on the radio, the way the pool glows outside my window at night, Wallace and Grommit, skiing, the Big C, kites, bridges, cinnamon toast crunch, Rubik's cube, chimps, my swiss army knife, making faces for cameras on amusement park rides, helping people with homework, Italian food, the deck of cards in my backpack, twix ...
I'm sure I could go on forever, but I've got a full day tomorrow and I've got to get to sleep. Looking over and seeing my warm bed and big fluffy pillows is one more reason I just can't help but smile.
I don't claim that my reasons are any good, or that they'll work for you, but I guarantee that you can find yours. It's just like dancing -- as soon as you believe you can do it and allow yourself to, you'll automatically be a thousand times better at it, more popular, happier with yourself, and a better person.