What you need to look for.
This is just a quick guide on what you need to think about.
When choosing a Lawn Mower you needs to decide what sort of grass you're going to be cutting, I know you may be thinking, Grass is grass... Right? Well no it's not but we'll cover that in another guide. For now we'll decide what sort of grass you have in your garden, now most people will have just slightly long grass as they keep it cut, so a normal Rotary mower will be fine, such as a Mountfield SP470 or such and such.
If your grass is say, getting pretty close to your knees then you will need to think about looking at a Mulcher as they will cut long grass and chop it up very small! That's what you want! You also want one with about 150cc Engine on it as that will have plenty of cutting power for that tough long grass!
Don't be afraid of a second hand mower either as they are often still great, just make sure you check the main things... Engine/Blade and Deck.
Start it up and make sure it runs smoothly, with no smoke (Apart from on start up)
Turn it off and tip it on its side (The exhaust side) and check that blade for cuts and sharpness.
And also if it has a metal deck check it for rust, as it might be rusting through.
I will go into more detail in my next Guide, but if you follow these basic steps you'll find yourself a great mower. :)