This is likely to be difficult, especially if any significant amount of time has passed after the stain was caused. This is also the reason why most people apply some sort of seal, even if it is oil or wax only. The first thing to do is to wash the surface first with cool water ( wets the wood and arrests migration) then with warm water (softens the fat), even including a mild detergent such as dishwashing liquid. If this does not remove it then isopropyl alcohol is very good for dissolving oils and fats - the difficulty is getting hold of some, and also to prevent migration further across and into the wood. Try a good chemist (most are not really chemists but only pharmacists). Repeated attempts will almost certainly be needed and it is quite likely that some discolouration will in any case remain visible in the short term. Gentle sanding with a fine paper may be beneficial.