Whenever my students come across new words, I ask them to look for them in the dictionary.
- to look after them
- to look at them
- to look them up
- to look into them
- No correction required
Explanation: Look up mean consult some book for a word, whereas look at, look after and look into mean see carefully, take care of and investigate respectively. Therefore, the correct form is look up.
Another example: Please look up this word in the dictionary.