Cat urine is said to be notoriously difficult to get rid of. This isn’t necessarily because of the one time “offense” but also because cats tend to return to the place they sense they’ve been to, so the urine smell is likely to get worse over time, if you can’t deal with it quickly.
The first thing to do is to try to remove as much urine as possible from the surface that has been affected. If your cat urinated on linens or various other fabrics, make sure you wash and dry clean them as soon as possible – multiple times, if needed. If your cat picked a hard surface as his spot, then wipe it down with a wet rag before the urine dries.
If the stain has dried already, then removing it along with the odor can be a little trickier. You can do it by using enzymatic cleaners to remove cat urine from carpet. These cleaners release enzyme cultures which literally “eat up” the active agents inside the urine, removing them along with the unpleasant odor they create. These substances are typically sold as sprays that you can use directly on the surface. You should be able to find them at pet stores and home improvement stores, as well as other places that sell pet food and accessories.