I don't know what brand or model you have, but let me explain how a lawnmower works. The air and fuel mixture burns in the cylinder to drive the mechanism, and when the engine is at an idle it is possible to adjust the air mixture. I'm not sure that you can do it but it might be worth a try. Turn the key and pull up on the starter rope and then turn the throttle halfway to see if the blade begins to turn.
If it does, you're lucky enough to have a simple carburetor and you can probably adjust the mixture to gain a little more power. You'll need a small screwdriver and a flashlight. Have the engine running and look for the choke. If it's not there, see if there's a clutch near the flywheel. If so, adjust it so the engine is running smoothly, but not so fast that you can't control it. If you can't get it to work that way, then you need a manual and a carburetor kit.