Hello Becca, It is a matter of practice. Learning initially the order of words in the sentence is the first step .Once you learn to construct a certain sentence and know where to place the subject, object and the prepositions, it will take then lots of practice including trial and error ,where a good teacher will be guiding you and correcting your mistakes. By reading,practicing and chatting a lot ,the correct structures will be memorized gradually and you will eventually find it easier to form correct sentences with less mistakes. The site recommended by Jerrod is really helpful ,I tried it as well and it is a good way to get e-corrected by Chinese natives or teachers. Good luck:)