While most milongas, practicas, and group lessons are welcoming, warm, and people are willing to forgive beginners a few mistakes, dancing in close quarters and with people you don't know well can be challenging. There is also a code of conduct among milongueros; knowing a few basics will greatly increase your chances of becoming popular to dance with.
Perhaps most importantly, respect your partner, whether he or she is someone you just met or someone with whom you've danced for years. Be kind and considerate and this will go a long way, even if you're a beginner. Beginners should come early and leave late and dance when there are only a few people on the floor. If you find yourself too crowded, try to move to the middle of the dance floor. Leaders, DO NOT use your follower as a battering ram! Nothing will make her want to stop dancing faster.
Learn to follow the Line of Dance at all times -- even in group lessons. Nothing messes up a group lesson faster than a few people thinking they're salmon swimming upstream. Circle the room as space opens up in front of you.
Be clean and well-groomed if at all possible. Dress up rather than down. If you're a guy and you sweat a lot, consider bringing an extra shirt to change into halfway through. Shower, gargle, brush your teeth. While most dancers enjoy a hint of perfume or cologne, put on just the right amount -- you want to smell nice for the person you're dancing with but not the whole room. Never refuse a breath mint if one is offered...who knows, your partner may be giving you a hint!
Pay attention to every cue when asking for a dance. Cabaceo (an Argentine term meaning to nod the head) allows leaders and followers to ask for or refuse a dance with relatively little risk of offending someone. If someone doesn't seem to be looking your way, chances are they're not dying to dance with you. On the other hand, it's sometimes rude to chat lively with someone and then not follow up with an invitation to dance. Even if you're not a skilled dancer, you may find that people would prefer dancing with you than sitting down for the whole night.
Lastly, learn as much as you can and dance as often as possible with as many different partners as you can. You will feel for a long time like nothing is happening and you're not improving at all, and then ("like water beginning to boil" one teacher in Korea told me!) things will start to happen, slowly, then faster and faster. Tango requires patience and perseverence, but it pays off: no dance is more rewarding than Argentine Tango. You will find as you improve that tango returns all the effort that you put into it...and then some.