Not even the english native speakers use complex words in a daily conversation, so don't worry that much about it. If you have some native people to talk with, what you could do is learn a lot of 'phrasal verbs' which are pretty difficult to use for us(the non native) and try to use them in the proper situation instead of using those words in any random case. Have a great weekend!

READ. A complex word also has a more specific meaning than a simple word - so you're less likely to use complex words. Unless, you are being very specific. ;) The worst mistake is to find a complex word in the dictionary and try using it in casual conversation. Keep listening for when people do use complex words, then use them once you've already experienced those words "in action".