There are beginner-to-advanced tutorials in sims creation at
Modthesims, including some meshing (the object tutorial has a quick beginner meshing tutorial, which might be the easiest to start with. Clothes and bodymeshes requires a little more know-how, but there are beginner tutorial for those, too).
Meshing tutorials depends a bit on the program you want to use. The game seems to like
Milkshape best, and most game tutorials are based on it. It costs money, though...
Blender is a free alternative, but might be trickier to understand.
Personally, I switch between Milkshape and 3D Max (which might not be in the beginner class of meshing programs, and is in a class of its own on price). Max for detailed meshing and meshing from scratch, and Milkshape for game preparations, simple body meshing or simple adjustments to a mesh - I really don't like the work flow, and I'm lousy at meshing from scratch in there.
There are other programs too, but I'm not familiar with most of them... I thnk there might be a list on Modthesims.
For in-depth tutorials about meshing you've got Google and Youtube. Depends on what program you want to use and whether you like written or filmed tutorials best.