The corner butt joint would be the very simplest joint to use for joining the sides of a box:
This requires very little in the way of tools at the most basic, in theory you only need one saw to cut your wood to length. Then you simply fasten the pieces together — this can be done with glue alone, and without the aid of clamps if necessary. I should mention that only glued this joint is not particularly strong, however for boxes it can be strong enough.
However sawing would leave a rougher surface on the cut ends of the wood that should be smoothed and this would really then require one or more additional tools, for example a hand plane or a powered sander of some sort (manual sanding is an option but it takes a lot of effort).
Even if the sawn edges are properly smoothed this sort of joint is not considered very attractive, but that said if you use attractive wood the box will have an inherent beauty from the material used.
You also need to consider the box bottom. Again the simplest option here would be to cut a rectangle or square of wood, hardboard or plywood and just glue it to the edges of the four-sided box frame that you made previously.
But again this is considered ugly by many people (particularly where plywood is used) and most box makers hide the bottom by insetting it into grooves in the sides. However, forming the needed groove or rebate/rabbet requires further tools, and some experience to do well.
You could get away with just sawing a smaller piece of wood that is exactly sized to the hole in the four sides and simply glueing this in place. Not the strongest option but can be strong enough, particularly for smaller boxes or where the box is not intended to take heavy contents.