You'll need to be signed in with your Slack account to leave a channel. When you sign in, you'll be taken to your "#general" channel. To leave a channel, it will need to be your active channel in Slack. This will open a small menu of channel options. This will remove you from the active channel. You'll be taken to your last active channel. You cannot leave the #general channel. You can find all of the channels you've left in this list. Click one to open the channel preview and get the option to join it again.
Sign into your Slack team site if you aren't already. Click the channel you want to leave in the left menu. Click the Gear button in the upper-right corner of the channel. Select "Leave #channelname." Click the "CHANNELS" heading in the left sidebar to see your available channels.