Im trying to make my own mesh. Ive already looked through all of the maxis sims 2 meshes and i couldnt really find one that i wanted. Im trying to make a yugioh duel deck box (basically a rectangle box). Im not sure whether its better to use milkshape or blender for this. Also ive looked through the tutorials on here but none of them say how to create your own 3d mesh without a maxis clone.

You'll have to start out cloning a mesh in SimPE, but you're supposed to switch out the mesh and texture with the ones you're making, so don't worry about it not looking exactly the way you want when you start. Changing this is part of the tutorial (probably comes in later steps, maybe you skipped reading those parts?)

You do want to clone an object with the properties you want the finished object to have (placeable on all shelves as a small object or as a large object, is it a picture, a bed, a chair, table, etc.) - some properties are editable later, but the closer you can get it to the desired finished result, the easier your job is going to be. If you're making a small box, don't clone a large 4-tile deco object, because the cleanup work is not fun. If you want your item to work like a table, don't clone a chair (won't work like a table even if it ends up looking like one when you replace the mesh).

There are probably 100+ square box objects already out there, so you could make things easier for yourself and find an already made square box in the rough shape/size of this one, and just recolor it.

If not, a simple 6-sided box is one of the easiest shapes you can make in a meshing program (I've yet to come across one that doesn't have a "make a box" option), so it depends which program you want to use (I use both - I find Blender easiest for meshing and UVmapping from scratch, and use Milkshape for simpler edits, bone assignments, and often if I can avoid using Blender for something. Depends on the project, really).

You'll also have to UVmap the box. Can be simultaneously easier and harder in Blender than Milkshape, although I think Milkshape could handle a box just fine. UVmapping a box is a good way to figure out how everything is put together. If you find a good tutorial it shouldn't be too difficult. There are several at Youtube.