Here's my $0.02.
There are two types of Portfolio Mangers that claim the titles as PM.
The first type is not a real portfolio manager in my estimation but rather a banker in disguised. Basically, these individuals don't care about where the market is heading or strategic in thinking. Rather, they follow the bank's allocation guidelines into which mutual fund they should allocate the bank's client money. These individuals may or may not be CFA charterholders and just want to make a decent amount of money until retirement or fund their lifestyle with the min. amount of work. They usually get their start with a little luck and perseverance
The second type is the traditional Portfolio Manager. Although an individual PM's philosophy in investing differs, they are usually well educated in macro and micro environments. Passion for optimization is their game. Most likely, these individuals have an economic and/or finance background. They start their careers as either an economist/investment analyst/researcher and feed their ideas to a PM. Eventually (if they are good), the economist/investment analyst/researcher ideas become the "go to" person in the office and they land the job as a PM (of course they work their way up as sr. analyst/sr. researcher/jr.PM).
Back to you question of how do individuals get their start as PMs. The first part is passion for investments. Read and keep your mind open. Second part, is hard work. Start anywhere you can gain experience in a particular finance field (ie. fix income, securitization, ibanking, research, etc..). Third, move laterally until you're in a investment firm. Fourth, work hard and become the "go to" person in your office. Fifth, be a bit lucky and opportunistic (sometime you have to make your own luck). Sixth, work harder than anyone else.
Hope this helps.