In WhatsApp for Android, for example, you can head to the main app menu (tap the three dots to the top right) then choose Settings, then Privacy, and untick the Read receipts box. It's the same on iOS, but you can access Settings straight from the bottom tab bar. With that done, no one will know when you've picked up your messages, but you won't know when your friends have seen theirs, either. On the same screen you can tap Last seen and then Nobody to stop anyone from knowing when you were last active inside WhatsApp. Again, this setting blocks you from knowing the same about any of your contacts, so you can't have it both ways.