You get a hang of these things by building stuff, as well as looking at works and designs of others. 3/4" (~19mm) plywood is thick enough for most furniture pr

You get a hang of these things by building stuff, as well as looking at works and designs of others. 3/4" (~19mm) plywood is thick enough for most furniture projects. You could use 1" dowel for the legs and attach them by making a hole through the plywood, sawing a small split to the leg, drive it through and tighten it with a small wedge in the split.