I don’t recommend starting weapons training until at least one year into your tai chi training. Personally, I learned the form first and practiced it for 5 years before switching to a school that also taught weapons. In comparison with my fellow students, most of whom had just finished learning the hand form, I picked up the sword form relatively easily.
It was easier for me because I had my footwork down pretty well. They say that all errors in the form can be traced to footwork and the alignment of the waist/hips. The footwork in sword and saber is much the same as the hand form with a few variations. So, concentrate on your footwork, and when it feels comfortable, balanced and you’re sure your stance is wide enough and that your footwork angles are correct—then you can take up weapons with relative ease.
Saber has fewer movements and is less complicated and less subtle than the sword form, but both have wide sweeping movements, some of which are done on one leg—so if your footwork isn’t stable first, learning the weapons forms will be an exercise in frustration because the additional few pounds the weapons weigh will throw you off balance. I’m not saying it can’t be done. I’m saying that it really does make sense to wait a bit…although had the sword form been available at my old school, I’m not sure I would have had the self-discipline to wait.