Not much to go on, but it's probably a Chiese engine.
If it starts, then cuts out after a few seconds (or when the choke is released), there are a cpuple of simple things to check. 1) Stale fuel. This can and does kill mowers - fuel left in the tank from last year is the mechanical equivelant of 3-month-old beer. Don't use it! 2) Water in the fuel. A single drop of water in the carburettor float chamber will effectively starve the engine of fuel.
In both cases, the answer is to remove the carb float bowl & flush the tank through with fresh fuel.
Depending on the model, the jet may be contained withing the bowl nut. If it is, check that it's clear. If not, the carb will probably have to come off the engine.
(phil_saunders(a)bigfoot.com)
Phil Saunders, June 2013
