Look at the verb that comes before it. That usually defines which form you need, though there are some words where the difference doesn't matter, and other words which take both forms, but with different meanings. Prepositions before gerunds, of course. Infinitives usually follow adjectives. If you search for "gerund infinitive English", you'll find plenty of lists and examples for using each one. You'll just need to learn them.

As Peachey said, search the Internet first and you'll find that you can often answer your own questions. http://grammar.ccc.commnet.edu/grammar/verblist.htm