I understand that there are five positions of the blues scale, up the neck.

Can someone explain to me, however, how to decide which "position" to use???

How does memorizing the boxes help ?? Thanks!!!
Memorizing the boxes (positions) helps because you can apply those all over the fretboard no matter what key you're working on. If you're in any given key, you can use any of the boxes of that key and it'll sound fine; you can use certain boxes to give a different sound though, like starting on a third when you're playing against a root chord or something, because the roots of the boxes are different.. It's up to you.
Well, the notes are repeated in each "box," so it doesn't really matter which one you start in. I guess just choose whichever one is the most comfortable to you. The boxes are just a simple way to learn the scale... once you are comfortable with the scale you can easily move in and out of each "box." They just break it up into 5 parts.
I understand that you are a beginner, that's cool. I'm just going to tell you not to be too dependent on boxes in your guitar playing career. It may be a good idea to practice with note positions nice and early. I've been playing for a few years, and am only now learning note positions. Its something that I regret not getting into earlier. In case you don't know, what I mean is learn the notes of the fretboard better than you know the back of your hand. Once you've done that, you can learn scale formulas. But that's not necessarily something that needs to be done straight away, I'm just suggesting that you might want to get into that early. Some people I know don't know the notes on the fretboard (like myself) and can play pretty well, so don't get worried about it.
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