MIG welding is a good process to start welding if you're a beginner. However, while it's easy to learn it's quite difficult to learn to do WELL. Mastery of the MIG process require learning exactly how to adjust both the voltage and the feed speed for any given situation.
100% CO2 is actually a troublesome shield gas which is difficult to use and generally produces poor results. I would strongly advise a beginner to start with Argon/CO2 mixes.
->TIG is more difficult than other processes. It also requires a person to learn a number of skills and techniques which are unique to TIG and not really transferable to other types of welding.
->You will need 3-4 years of experience in MIG, TIG, and stick(SMAW) before you could realistically be able to pass a 6G weld test on pressurized pipe. Pipe welding takes lots of experience.
-> Submerged arc welding (SAW) is an obsolete process which is a waste of both time and money for the companies who still use it. Tragically, many weld shop managers cannot do basic math and calculate the cost of one meter of weld. Don't even bother with it.
-> Plasma cutting is easy and anyone can do it with about an hour training.