contact set

bonded contact  = welded or glued

but welds are a really complicated topic, there may be combination between no penetration and bonded depending on how you are "welding" things. but start with bonded.

mesh type is based on the type of part, blocky part = solids, thin part = shells, long slender = beam

element size, start coarse and refine as needed, see topics on convergence

You need to be careful with how much relevance you place on FEA results where a weld takes place; as Jared pointed out, welds are complex joints. Not only will the material properties differ (due to the mixing of the base metal and the filler, phase changes in the base metal due to the high temperature),  but you'll also have residual stresses around the weld. Most importantly, if your weld doesn't have full penetration, then you need to capture fracture mechanics.