How do I start Telegram from the command line?

Since you installed telegram from the PPA, it is likely installed in /opt. Probably under /opt/telegram. You should be able to launch it from the command-line by running:

/opt/telegram/Telegram

but if you wanted to make this easier you can create a symbolic link to /usr/local/bin like so:

sudo ln -s /opt/telegram/Telegram /usr/local/bin/telegram

you can then launch telegram with the command:

telegram

On Ubuntu 17.10, 18.04 and above, If you have installed using Ubuntu Software Manager, you can launch it using the following command in terminal:

telegram-desktop

if you want to start it minimized in tray use the following command (e.g you want to create an start up entry):

telegram-desktop -startintray