How viable is building your own bike for a beginner?

I’ve been looking at buying a new bike as the one I have now is not fit for mountain biking. While looking at bikes online I became interested in the idea of being able to pick out all the part that would fit me. So how viable is building your own bike for a beginner with very very little experience in bike mechanics but is willing to learn? Ok, bonus question. If I do choose to build my own bike what to the best place to buy parts? [color=#333333]Speed Test[/color] [color=#333333]Scrabble Word Finder[/color] [color=#333333]Solitaire[/color]