Leaking battery can be a serious problem. Fix it very carefully, otherwise there is a risk to provoke a nasty chemical reaction.
Need to be concerned about their own safety. Rubber or latex gloves to protect hands. If we are talking about the battery or lithium battery, it does not hurt and goggles. AA batteries are alkaline, lithium, Nickel-cadmium and lead-acid. The last two types are dangerous: they can flow hydrocyanic acid, are able to do a hole through the metal.
Neutralize it with baking soda. Alkaline battery is best to wipe with vinegar, fit and lemon juice. Lithium (for phone battery, too) is removed with plain water.
If the battery stood in some device, it will have to be cleaned and electrical contacts. Spilled liquid to gently scrape the floss, the contacts gently wipe with a damp cloth.
And best of all will this situation be warned in advance. Do not put in the same device of various batteries. To get the batteries from devices that are not going to use for a long time.