Welcome!

First a simple one, YGM means you've got mail. For example, if I were to send you a private email, I might enter YGM in the thread to alert you that you have mail. Many members of the Flyertalk community/message boards use a different email address for Flyertalk, partially for anonymity. People might send a private email for a variety of reasons like meeting up at some airport, trading coupons, or to say something they might not want to announce out in the open. Something that you might not announce in the open would be an event like, you bought a cheap $200 ticket from JFK to London, and a friendly agent double upgraded you from coach, to business, to first class not requiring any miles, points, certificates, etc. This is a bit of an exageration, though not impossible, but you get the idea. There are travel industry people that monitor the board, and fear is that any loophole or good deed may get reported if posted.

Getting back to basics, you should study which airline you would likely fly the most. Things to consider in pciking an airline to fly the most with are among other things:
For example, what city are you in? Where do you fly to most often? Do you buy cheap or full fare tickets? How important are upgrades to you? Pick a few airlines you might fly the most with, and then review their forum on Flyertalk. For instance, the Delta forum, the British Airways forum, etc.

You can join most any frequent flyer plan on the given airlines web site. This is a very simple process. Similarly, you can join a hotel guest loyalty program on the hotels web site.

Once you fly a lot, usually 25,000 miles or 30 segments in one year, you become what is known as an elite, preferred, or premier member in an airline program. E.g. you are a top customer. There are usually three levles of elite, preferred, or premier membership. E.g. Silver, Gold, and Platinum (each airline calls these lvels something slightly different). You generally reach the second (mid tier) level by flying 50,000 miles or 60 segments, and the top tier by flying 100,000 miles or 100 segments. Recently, however, some airlines (namely Delta and Continental) make distinctions about not only how much you fly, but how much revenue you spend. This is only very general conceptual information, you will need to review the nuances of whatever program you want to focus on.

When you reach the elite levels, you get certain published and undocumented benefits. For example, free upgrades, bonus miles, and special faster lines to check in at the airport. If you are stuck in coach, as an elite flyer, you often can choose from among the coach seats in the front of the plane. Also, largely depending on airport agent discretion, you can get some undocumented benefits by being an elite member of if the agents just like or recognize your face. For example, they may upgrade your friends traveling with you, they may give you more generous upgrade benefits than the rules technically allow, they may waive the standard change fee for making itinerary changes, or they may give you a hotel voucher if you get stuck in Chicago due to a snow storm.

One other thing you will read about is a mileage run. People on this board value their status, probably more than most people out in the population. So people sometimes take trips solely to earn miles and keep that elite status we talked about above. Also mileage runs can be taken to quallify for certain promotions. In a mileage run, for example, a passenger might fly from JFK to London, spend only 2 hours in London, and then return back to JFK. As you can imagine, in these times of security hype, this can raise a few eyebrows though there is nothing illegal about it. Also some people take extra segments to get more miles, and to help them quallify for elite status. For example, instead of flying on a JFK to Los Angeles nonstop flight, a person might fly JFK, Washington, Chicago, Denver, Los Angeles. Usually this routing thorugh several cities costs just about the same thing as a nonstop, except you can pay slightly more in taxes.

The degree to which you want to get involved in these mileage programs is up to you. You can just join a mileage program and give them your number when you fly, or if you really get the mileage bug, you might be doing mileage runs and crazy routings to keep or move to the next elite level.

Welcome, and I'm sure others will give you some information as well.