As adjectives, both mean "inside", although they collocate with different types of nouns. Alan's suggestion of looking up some sentence examples in dictionaries is a good idea to help get familiar with which nouns they typically modify, respectively. "Interior" can also be a noun, while "internal" can't (it's always an adjective). Also, "internal" has a corresponding adverb, "internally," while "interior" has no corresponding adverb.

You can use a dictionary, look at the definitions and the sample sentences. That way you are not so dependent on other people for your language learning. http://www.oxforddictionaries.com/definition/english/internal and http://www.oxforddictionaries.com/definition/english/interior If that dictionary is too difficult, there is a link to the Advanced Learner's Dictionary on each page.