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The first thing you must do is remove the blade from your lawnmower. Before going under your mower, be sure that the spark plug wire is removed from the spark plug and isolated from it(as far as possible). The blade is usually attached to the motor with a fine thread bolt that has a 9/16" hex head. This means you can use a 9/16" box end wrench or a socket to remove the bolt. You may be able to hold the blade with one hand and loosen the bolt with the other. For stubborn bolts you may need to put a piece of wood through the deck chute and turn the blade until it is locked against it. Once your blade is off, you can take the blade to any repair shop and they will sharpen and balance it for 2-7 dollars. If you have a grinder, you can do it yourself. For a handheld grinder, clamp the blade to the bench top or in a vice. Follow the existing angle and remove material until the edge is sharp. A simple way to check the balance is to hold the blade out with a pencil in the mounting hole. If the blade seems to fall to one side, more material must be ground off until balance is achieved. When you re-install your blade, be sure it is not upside down. The side that is ground should face the motor.
The answer is very simple. Disconnect the spark plug wire first. Now find one end of the blade and mark a spot on the deck where the bottom of the blade lines up. Now rotate the blade so the other end lines up with your spot. If it lines up above or below your mark, the blade is bent. Don't try to straighten the blade, replace it. You can normally get a new one in most department or hardware stores.
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The reason is most likely stale gas. Gasoline, in small amounts, goes bad very quickly. Also when it evaporates from your carburator, it leaves a residue. Drain the existing fuel as much as you can and top off with fresh gas. If you still can't get started, try removing the air filter and covering the hole with your hand. When you try to start it you will be giving it a full choke. Do this only once or twice. If the motor doesn't start, let it set for 15 minutes and try starting it with no choke. If it still won't start, submit a question to me.
If your mower has had a lot of use, oil could be getting by the piston rings and getting into the combustion chamber. In this case the motor needs to be rebuilt or you could get a new short block on my parts page. The answer may not be this extreme. Have you been cutting on a hillside or in a ditch? If you have, oil has seeped into your combustion chamber. Leaving the motor, run on a flat surface, until it clears up(5-10 mins.)should do the trick. Another possibility is that you have overfilled your crankcase with oil. Drain your oil down to the proper level and run until smoke clears up. If the engine won't start, you may need to remove the spark plug and clean it. I find that carburator cleaner works well, but let the plug dry before you put it back in.
Most walk behind mowers have a drain plug under the motor. It may be hidden under grass, so you will have to scrape the bottom of the motor off to find it. Agian remember to disconnect the spark plug before going under the mower. The head of the plug is usually square and about 1/2". A 1/2" open end wrench, or adjustable wrench should do. you will need a thin pan to drain the oil in. On newer Briggs & Stratton engines you can remove the dipstick and tip the mower towards the dipstick. Put your foot in front of a wheel and tip it towards you. The trick here is not missing your drain container.
Everytime you use your machine. I can't stress this enough. Motor oil is much cheaper than a new motor. Even if it didn't blow before you added oil, you still have done great harm to the motor. All motors should use some oil. Even new ones, especially before they are broken in.