By "tonic" syllable, do you mean which syllable is stressed more than the other syllables. When you learn a new word, the first thing you learn is how to pronounce it, including stressed syllables. There are a few rules for this but the easiest way is to use an online dictionary. Most of them will pronounce the word for you. You can also go to www.howjsay.com and type the word in to hear it pronounced for you. Once the word is pronounced, just pronounce it the same way.