Use a bootable USB flash drive or optical disc with any live Linux distribution loaded into it. For usability, and user-friendliness, I'd suggest Ubuntu but the USA's Airforce developed and release their own distribution called LPS specifically for such use-cases.
If you prefer Microsoft Windows and have a an Enterprise licence, you could try Windows To Go.
You should still check for hadware keyloggers (follow the keyboard cable to the motherboard and look for anything plugged in between both).
Something I did when using public computers that I didn't trust, was to type broken passwords, that is: If you password is "password", type "word" then with your mouse click back to the beginning and type "pass". If the keys are being logged as plain-text, which it usually is, instead of seeing
password[enter], the spy will see:
Although going a little further than the scope of the question, a password manager will allow you to use more secure passwords (with a master password) meaning you won't be typing as much sensitive information into unsafe computers. Do be mindful that your weakest point is then your master password which should be changed regularly.