I strongly recommend reading up as much as possible on Tango etiquette.
Some basics can be covered in class, and yet the onus is on you to do your own research.
Argentine Tango has expectations of behavior on and off the dance floor. These are not complicated, or esoteric, and all point back to the basic idea of respect. Act like a gentleman or a lady, and you pretty much got it.
Personal Hygiene - you know you need to smell clean. But you might not realize that colognes and perfumes are often too much. You are only a few inches from your partner. Scent yourself very lightly or not at all. Carry mints. Use them. Share them.
Use the Cabeceo (The Nod - The Look) - not only does this technique create a respectful environment for both of you, it also respects the traditions of a dance that is steeped in a culture and history. It helps both of you save face if a “no” is on the table. Women ask and seek for dances just as much as the men do. No one is obligated to dance for any reason. Ever.
Talking - avoid talking while dancing. Tango is complicated and involved - it’s hard to focus when someone is chatting.
Teaching/Helping - Never - Ever - “give advice” during a milonga. Always ask before offering advice if you are in class or practicing.
Receiving uninvited criticism is possibly the number one reason why most beginner dancers leave the Tango community.
It is considered inappropriate and very rude. If you need to communicate something that is not working, use “I need” or “I feel” statements.
It’s always a good rule to pick out something that is working and complement the person on that at the end of a dance or tanda.
Floorcraft - this is much like driving on the road. The Tango follower is aspiring towards a “zone” that is pleasant and comfortable. Zig zagging across lanes, tailgating, wild movements, crashing into other dancers; these all create a feeling of distrust and discomfort not only for the person you are dancing with, but also everyone else on the dance floor. Avoid racing, dance within a lane, take care of your follower. Followers, you are responsible for controlling your adornos/boleos to avoid injuring your partner and others on the dance floor.