There are two modes of chat in the game, each with a different filter. To chat, press the "/" key on your keyboard. This will open a small bar at the bottom of the window. Type what you want to say, and press Enter.
You can also Copy & Paste. To do so, copy your text (e.g. from a word processor), press the "/" key, and then press Ctrl + V to paste in the chat-bar.
ROBLOX also has an animation system that allows Emotes. For more information, read here: Animation System (Emotes)
For more information on Chat Modes, read here: Age, Chat Modes, & Filtering