What should I put on my face before using a tanning bed?

If you need to protect your face during tanning, e.g. because of an existing tan or because of sensitive skin, there are multiple ways you can protect yourself. You could use a light, moisturising SPF 15 sunscreen; this would prevent some, but not all, of the UV rays tanning your skin. Alternatively, you could cover your face with a towel or some other fabric which would do the same thing.