The eyes are one of the most important parts of being L. He has bags under his eyes from nights without sleep, but it's not recommended you stay up all night due to health hazards. Wearing some black eyeliner and eyeshadow should work if you do it right. Another way is to wear mascara to bed, especially on the lower lashes. When you wake up in the morning you will have at least grayish bags under your eyes. Aside from this, your eyes need to have almost a blank look to them and make you look a bit lost. Always stare at people when speaking to them, your eyes should be wide open yet it shouldn't look forced. Try not to let your emotions show through your eyes. It needs to look like you know something others don't.
Your hair needs to look messy. Spike it if you really want to, but don't make it obvious that you did this on purpose. It should look like you've just gotten up from bed. Add a bit more hair to the left side of your head when styling if it isn't cut like that already and make everything look uneven. Hairspray helps. Fluff it up and put some hair over your eyes.
The ends of your hair shouldn't be much longer than your neck. To add to the look, check and see whether the left side is longer than the right. If it is, you're doing pretty well so far. If it's not, that's alright because most people won't notice. If you don't have black hair. If someone tells you he has blue hair, they're wrong, he has blue hair only as an "accent" opposing the aura of red they place in Light Yagami. Black hair dye will suffice.
If you don't want your hair to be black forever just use the temporary kind that washes out in the shower. If you have curly hair, just straighten it. Or if you'd prefer not to dye and cut your hair, another option is to buy a wig and cut and style that instead. It may cost a bit more, but you'll have it in case you ever choose to be L again.
L's clothing is simple and cheap. Simple baggy blue jeans are a must, don't wear a belt with them. Try not to spend a lot of money on your outfit. $100 jeans aren't exactly L-esque. L's long sleeved white shirt is a little big for him, not too small.
Do not wear any accessories. No shoes, socks, glasses, or jewelry. If you must wear shoes, try wearing sneakers or white flip-flops. The more worn-out they are, the better. Socks aren't recommended. At college he wore sneakers, but no socks.