Second step is to explain to us exactly what you want to do. Not just "morher board repair", but what do you really want to do? Get a job doing it? Start a business? What background on this do you have? Etc....
There are on-line resourses (classes and vids) but they will not replace competent hands on teaching. As Tiger points out, your current knowledge level can help us guide you to a starting point.
I will suggest looking for and reading articles like this one http://people.richland.edu/dkirby/mbdfaults.pdf "Diagnosing And Fixing Motherboard Faults" as a start.
First must determine cost effectiveness.
IE a $400 (Expensive MB). Divide by 3 = $133. Then 1333 / $25->$35 (good wag for a "certified Tech”) to, With Proper test equip, diagnose Problem = approx 4.5 hours, then ad time for an individual qualified to solder on a multilayer MB. If time spent exceeds the 4.5 Hours - the MB would be classed as Non-repairable and pitched. For a typical $159 MB this time is down to 2 Hours.
This excludes most repairs outside of a "simple" diagnosed problem.
2). Soldering. May techs think they can do a "good" solder joint, Most would never pass a class on "high reliability soldering" for a multilayer MB requiring replacing a soldered in IC. (I was Certified for many years, two week class + recert ever two years.
3) Test equipment: A cheap Digital Multimeter will not hack it. I have (at Home a quality DVM (cheap is OK) but also have external Powersuplies and an expensive digital O'scope - and that is not sufficient to diagnose many of the problems.
4) Electronic Knowledge, More than just knowing ohms law is required. Knowledge can be obtained thru self-study, but also requires real hands on diagnosing defective circuits.
Bottom line, if this is just for Home, repairing your own MB. And RMA is out, sure you can try it, but outside of a simple fix. More than likely you will create more problems than you will fix.
If you have a defective motherboard and that's the reason for this post be aware that repairing motherboards requires both the proper hardware, knowledge (skill and suitable reference material for chips/motherboard specs etc) and parts. It's not feasible for most people.
I have an electronics diploma, have repaired circuit boards but I can't repair a motherboard.