Best way to look intelligent- don't talk very much. The more you talk, the more likely you are to look stupid. If you are intending on sounding smart, then you actually have to become smart. Otherwise everyone knows you're faking.
Work on grammar and spelling. For instance use capital letters in the beginning of questions and sentences. Next use singular and plural verbs correctly. I suggest paying attention in English class. If you're out of school take one at a local community college.