The first thing you will want to check is the lint trap. If the screen is full, clean the lint off and re-insert it. If this doesn't solve the problem you may need to clean out the lint trap duct. On some dryers this can be accomplished using a vacuum cleaner with the narrow attachment that's used for vacuuming along corners.
Also, re-check the lint trap screen. Chances are it's clogged by all of the chemicals we use to clean our clothes such as detergent and fabric softener. You can check this by putting it under the faucet and turning it on low. If the water beads up on the screen, you need to clean it. You can clean it with ordinary liquid hand soap or liquid dish soap. Apply the soap directly to the screen and rub it in with your hand until it sufficiently cleaned.
Another problem could be the vent duct running from the back of the dryer to exhaust vent that runs outside of your home. If this is clogged with lint, you will either need to clean the duct out or replace it. You can choose to replace it (which is highly recommended if there are any holes in it) use a short piece of duct as possible to get from the dryer to the exhaust port. the longer the duct is the more you increase the chance of it clogging later.
Check the exhaust portal outside of our home. If it is clogged clean it out by hand. If birds have built a nest in it, remove the nest and cover the portal with a wire screen to prevent any more feathered friends from setting up a homestead.