My pillars of fencing, maybe in some order.
possibly.

Measure.

Maintaining and understanding distance. Getting your own is easy, you can just walk near a wall and see. It doesn't really change that much between "normal" weapons. Rapier distance is a touch further than longsword. But as the endpoint is sword and arms outstretched with a shallow lunge, it's mostly moot and relatively unchanged. Once you know it you know it regardless pf what's in your hands.

The real trick is getting your opponent's Measure. Are they all arms, or all body. Tall matters a lot less than long. Is their blade particularly long or short for their build. If you can casually stroll and stop just out of distance you can countercut without much extra skill required.

Open and closed lines

Recognising what is open and closed is another key skill. This is spotting where your opponent is vulnerable. Not too complicated at first, but it's common for a false opening to be presented. Most guards invite an attack to an easily guarded location and second intent comes from knowing where the riposte is coming from.

Tempo

This is speed/pattern of play learning when to strike. Cappo Ferro has a non-exhaustive list of times, but it boils down to when your opponent has committed to motion. Contra-tempo attacks like stop thrusts and counter strikes are my preferred style. But that requires good Measure.

These pillars keep you safe long enough to get good and provide a sound foundation for whatever skill or style you wish to develop.

Play becomes practice

The penalty for failure is, at worst, a bruise or two. It's not life or death and most of the time this is a collaborative endeavour. It is nerve wracking to have someone come at you woth a sword, even when you know all the safety equipment and methods. But being tense makes you slow and stupid. Your muscles have to relax before you can move. White knuckles will cause fatigue, and you'll lose your sword. This is fun, and although you can be serious, you shouldn't take it seriously. We get more skills through play than stony faced determination. Mistakes teach us more than success and matter how good you are, you can always learn more.