What you need to do is get out to one of their events. Even though the person who told about their product/service may be a flake, they will always have an event where a 'big speak' (maybe a CEO or other high-ranking company guy) will be talking at. Go to the event, talk to that guy (you'll find most of them are very approachable and intelligent) and ask your questions.
I would strongly recommend staying with products/services that you would personally use and get great benefit from. There are MLMs for just about everything. You need to make a list of what you would like, do your due dilligence (online research) and contact a local rep.
I did this when I joined Mannatech
. I looked up "scam +( insert your mlm of choice)" in Google to see if they were legit. I even found a site where someone was bashing Mannatech. Upon further reading I found that he was bashing the company because his sister-in-law was making wild claims (both illegal and against company policy) but this guy chose to blame the company.
I am no longer active with Mannatech. They are a good company, I visited their complex in Texas 2 years ago, pretty impressive. I have moved onto other area in my life and Mannatech no longer fit where I needed to put attention. Heck, I was even making money and still left. You must choose wisely because it will be a lot of work and if it's not something you would put a lot of time into, you'll get discouraged and depressed real fast. Think of MLM as a hobby that you want to do all the time. You sleep/eat/drink(in many cases literally) your new found hobby.
You also must realize that no matter what you choose, regardless of what their salespeople say, your product is NOT for everyone. That's the biggest mistake most of them make. They pester everyone they know, telling them they simply have to get 'this stuff', and when that doesn't happen, they get upset that 'it doesn't work'.
I believe that network marketing is the BEST way to learn how to sell. You will learn quickly what works and what doesn't work. And what better way to have the Gov't cover many of your costs come tax time(because these are legitimate businesses and thus have many deductions associated with them), so it's a nice bonus
You also learn quickly how to run/not run a business. Like anything else, it has to be systematized and organized.
So, back to the beginning. Start with what you like to do. Find some companies that promote products in that category, then see where they hold meetings/events. Consider local business networking groups. There is always MLM people there.