The ketone functional group looks like this:

Molecules with the ketone functional group will have the suffix -one. For instance, in this molecule:

This would be 2-pentanone. You must number ketones, unlike aldehydes, since the ketone will not be located at the end of a carbon chain. Like always, you try to get the lowest number possible.

In terms of functional group priority, ketones take priority over everything but carboxylic groups and aldehydes. So, among all the functional groups that are no located at the end, ketones have highest priority.

If an aldehyde or carboxylic functional group is presenta long with the ketone, the ketone becomes an oxo- prefix. So, in the case of this molecule:

This would be called 2-oxopropanal.