明日（は）映画に行きます is fine, but rather colloquial. As far as I can tell, proper Japanese should be: 明日（は）映画を見に行きます　（あした（は）えいがをみにいきます）. Literally it means: "Tomorrow I'm going to go watch a movie". This sentence contains a very useful grammatical structure: ます-stem of a verb + に + 行く (or 来る), which you can use every time you need to say that you're going somewhere to do something. Examples: 友達と飲みに行った (ともだちとのみにいった) "I went drinking with friends" 新しい服を買いに行く (あたらしいふくをかいにいく) "I'm gonna go buy new clothes" 遊びに来てください (あそびにきてください)　"Come visit me"

Yours was almost correct but you have to add the"は" in the middle. Corrected sentence: あしたはえいがにいきます。 -わかったね！^_^

Perfect. I don't see anything wrong with your sentence.