Contrary to what you may think when you're five and playing with Barbies, everyone's legs (in fact almost no one's) look like Barbies: free of stretch marks, cellulite, and unwanted "extra." Some legs, due to either lack of exercise, nutrition, or just that's the way they are, are thicker than others' and you want your clothes to flatter your legs rather than accentuate their less attractive qualities.
The first thing to remember with thick legs is if you really want to do something to improve them, start eating well and exercising and you'll likely have pleasing results after a while. Until then or if exercise doesn't quite suit your liking, there are a few tips to making thick legs seem not so thick. One of the most important guidelines when dressing is to wear clothes that actually fit. Too tight is not a good look at all and too big doesn't mask your legs but rather makes them appear bigger than they are which is another thing you most definitely want to avoid.
When wearing trousers, wear wide leg trousers that don't accent your thighs. Wide leg pants make the leg all one size and is way more slimming than wear closely fitted pants. You can wear boots with pants to get the look you want (however do avoid pointy shoes), but never wear boots with skirts or anything that actually shows the leg. I was walking to class one day and this one girl with pretty thick legs was walking toward me and I did a double take when I walked past because she was wearing this skirt with these huge, hairy boots that made her legs look huge. I'm sure her legs weren't as large as the boots made them appear, but that was a highly unattractive look for her and I'd advise against that for anyone with thick legs. Remember, thick legs aren't bad; you just have to know how to dress for them.
Don't wear tights or anything that makes your legs look thicker. Because thicker is the look you're trying to avoid, don't wear anything that could accentuate that. Also avoid shoes with skirts and shorts and such that cut off the line of your leg (e.g. ankle boots). To combat thick legs, you want to make your legs appear longer to offset the thicker perspective; if you cut off your legs at the ankles your legs will only end up looking thicker. That translates into a very bad idea!
Thick legs aren't the end of the world and you can always work with them to still succeed in creating a styling look for yourself. Just know how to dress for your legs and you can always be a knockout!