How to use the four most common types of welding

I don't know anything about welding, but having a pal around who can tack a couple of pieces of metal together is always handy.

A few weeks ago a couple of pieces of metal that were screwed together, inside my pizza oven, fell apart. Looking at the problem, I can see that welding is my solution. As my former expert welding pal has disappeared, I am searching YouTube and watching a lot of videos before I go and find someone else!