This is actually an excellent question, unfortunately, there is no real "good" answer for it. Beginners usually start on a very light reed because the muscles that make up their embouchure haven't developed yet. The advantage of a light reed is that you can get a very quick response out of it, especially in the center register. The disadvantage, is that this free response causes the tone quality to suffer.
I usually advise that a student play on the hardest reed they feel comfortable on and can play for long periods of time on. Over time, this will build up a strong embouchure and playing habits, and the player can actually move down to a lighter reed, because they'll then be able to get a better tone quality on the lighter reed.
My best advice is to have a good teacher to advise you specifically on where you should be. I usually recommend something like a Rico 2.5 to start on, after a year or so time maybe moving on to a 3 or a 3.5. I usually don't go harder than a 3.5 unless there has been several years experience, in which case I might jump to a 4, and then work downwards to a 3 or 3.5. It's tricksy. Play too hard of a reed and you'll develop bad habits, play too soft of a reed and your tone will always be lacking, your embouchure won't develop well, and it will be harder to learn certain advanced techniques.