If your chainsaw starts to cut crooked and or cuts very slow it most likely has come into contact with something that has damaged the cutters like a rock. If the chips coming off as you cut are not nice and big and more like sawdust it will be time to sharpen your chain.
For this you will need a round file and a file guide both matched to the manufacturers specifications for the chainsaw chain you are using. The file guide fits over the chain with the cutter exposed and a slot at the proper angle to file through making it easier to keep the proper angle. Usually about 25 degrees as the most common and up to 35 degrees for more specialized uses. The file guide will also keep approx 1/5 or 20% of the round files diameter above the cutters top plate.
File the chainsaw chain from the inside of the cutter to the outside. File all the cutters on one side then the other equally until all the damage has been filled out. The top plates of all the cutters need to be kept even or the chainsaw will not cut straight. Always inspect your chain for damaged drive links, broken cutters or side plates or loose rivets. If any of these problems are found take your chain to a dealer for repair or replacement. Operation of your chainsaw with this type of damage could result in serious personnel injury.
Keep your clutch and drive sprocket area clean and clear of any debris. Keeping up with all recommended maintenance recommended in your owners manual, clean air cleaner fresh spark plug ect will ensure trouble free operation.
Remember to alway read your owners manual, beeing sure to follow the recommended safety precautions and always wear protective gear when hadleing a chainsaw, including sharpening one.