As others have mentioned, you can use the Messages app. However, if you're trying to send SMS texts (standard phone texts) then there are some prerequisites: your phone must be an iPhone, it must be running iOS 8.1 or later, and your computer must be running OS X 10.10 or later. If those conditions aren't met, then the best you can do from your computer is to send iMessage-based texts, which can only be sent and received by other people who use Apple devices.