An idiom is any expression which has a non-literal meaning. For example, if you say 'Can you give me a hand?', this is an idiom, because you don't literally want the other person to give you a 'hand' : you want them to help you. If you something is very easy, you can say 'It's a piece of cake' - this is another idiom.
A phrasal verb is a combination of a verb plus one or two other little words, usually adverbs or prepositions. When they are combined, they take on a new meaning. For example, 'come' + 'across' = 'come across', which means to find or meet.
NB A piece of advice: Don't write 'wanna'. If you want to write proper English, you should use proper words. Take a look at this: http://www.italki.com/question/307087