I have to put the appropiate word down. I don't know what word to use though.
how do you know what appropiate word to put in a sentence.
I have the same question often (and not just in Spanish (In general, as you did not provide a sentence with which we might help you in this particular case) I look up all the words that come to my mind to choose and read the example sentences in the dictionary. Sometimes I even do that using a thesaurus in English.
You really need to understand the rest of the words in the sentence to be able to provide a good word to plug in. Make sure you look up any unfamiliar words and understand what the statement is trying to convey. Usually if the meaning of the sentence is known the word you need to plug in will be fairly obvious.
What's the sentence?