It only takes a minute to begin dancing the Argentine tango. This is because the basic tango rhythm is SLOW-SLOW, stepping on every other beat of music. And because tango is based on walking. We are all expert at that.
The most-used basic tango pattern is the two-step walk. If you're a woman take two steps back, beginning with your right foot. If you're a man take two steps forward, beginning with your left foot. This is el Paseo (the Stroll).
Now do it again. And again. And again - any number of times. You can do this because the Paseo starts and ends on the same foot. (ALL the two-step walks in tango - not just the Paseo - begin and end with the same foot. So you can combine the different two-step walks in any number and any order.)
Now put on some tango music. (Look here for how to get it, if you don't have any.) Or select any kind of music you have that has a steady medium-tempo rhythm, or one with an irregular rhythm which is quiet enough that you can ignore it. Take your (real or imaginary) partner in a ballroom dance embrace. Don't instantly start moving. Focus on your partner and your embrace. Adjust your hold, enjoy the feel of your arms around each other, relax.
Wait for the right time to begin moving. If you're a woman, that means when you feel your partner begin to move. If you're a man, it means when the rhythm of the music begins to take hold, and you feel ready. Not before. Then begin dancing, with your head up, alert to your surroundings, your partner, the music.
Move in a straight line at first. Than as you (the leader) begin to feel comfortable, and you feel your partner becoming comfortable, begin varying your path. One way is pivoting to the left or right, something you do all the time when you just walk. Just as in ordinary walking, you can pivot before a step, or after it, or before AND after it. If you go pretty much in a straight line (maybe with a few S-curves), you are doing la Caminata (the Walk). If you keep pivoting to the left (or the right), you are doing el Circulo (the Circle).
If you get asked to dance, tell them that all you know is how to walk. If they still want to dance with you, take them up on it. Chances are they will try to do something you don't know. But don't let that bother you; you already warned them and it's their fault, not yours. And, who knows, some of it might stick with you after the dance.
Just relax and have a good time. Don't worry about all the fancy stuff you've seen in movies or shows or on the floor around you. If it all comes together and you're having a good time, you're doing authentic Argentine tango. And all the people who are frowning and struggling to do incredibly complicated figures are only faking it. Because real tango is in your heart, not in your feet.