How can I make sure my cookies wont burn?
Set the oven at the correct temperature. If the recipe says to bake them for 10 minutes. Turn the baking sheet 180 degrees at 5 minutes, and check them at 8 minutes to see if they're done. Then keep checking every minute or so.
If the bottoms are burning, double up on your cookie sheets by nesting one on top of the other.
Sometimes ovens run hot, so you might want to get an oven thermometer & check yours. Then you can adjust the temperature accordingly. Dark cookie sheets bake cookies faster than light ones. So if yours are dark, you can lower the oven temp by 25 degrees to compensate, or just check the cookies sooner, as Amanda suggests. Lastly, I love silpat liners. They seem to help cookies bake more evenly, with no charred bottoms or edges:
Use the silpat. Which is a good suggestion as mentioned above.
There is a human tendency when baking cooking to leave them in until the tops are done to the texture you normally expect from a cookie jar. Avoid that and err on the side of undercooked. (it's really against your instinct for testing doneness but try it)
When they cool they have carry over cooking time...if you just go by the tops the bottom will be burned, and the cookie overcooked.
Cookies should always be baked on racks placed in the middle of the oven. If you are baking on two racks, position them so that they're in the middle third of your oven. Stagger the baking sheets on the racks, i.e, one sheet on the left side of the top rack, another sheet on the right side of the bottom rack; rotate them after 8 minutes.
As advised above, use shiny baking sheets lined with Silpat or parchment (I prefer parchment). If you own dark, cheap, non-stick baking sheets, keep them but invest in Silpat-brand (and only Silpat brand) pan liners. Or invest in restaurant-quality sheet pans, which really aren't that expensive, and use the old ones for serving trays.
(The position of oven racks makes a big difference in how things bake as far as browning and doneness are concerned--it has to do with the position of the heat source and the flow of the heat, and any obstacles to the heat flow. Cakes, cupcakes and muffins should also be baked in the middle of the oven, where they receive equal amounts of top and bottom heat. Loaves of bread, pies and pizza--which tend to brown on top before the bottoms are done--should be baked on racks positioned at the bottom of the oven. Meringues and biscuits should be baked at the bottom of the upper third of the oven.)
I think Sam1148 is exactly right...I look for a golden brown color on the outer edge of the cookies, and then I pull them. I like to lift the parchment or Silpat straight from the pan and place the entire sheet/mat directly on the cooling rack. (Make sure you keep the sides taut while you're moving them, or you might end up with a few broken cookies!)