I know this is pretty basic info, but I figured newbies, like myself, are always looking for info on bikes. Plus, we need some good information pages here.
Filling It Up
Filling a motorcycle's gas tank is not like filling up a car. You can't just shove the nozzle in as far as it'll go and grab the handle. With most bikes, that will result in a tank filled about half way.
The way it needs to be done is a bit more involved. If you have a centerstand, park the bike on the centerstand to get the tank as full as possible. Once you get more experience, it is certainly OK to use the sidestand while fueling. Then, open the gas cap, before you grab the nozzle off the pump. Most motorcycle gas caps are relatively complicated, and you should open it first with both hands free the first few times. With many bikes, there will be a little flip-up cover over the keyhole. When flipped up, that allows you to insert the key, and twist to unlatch the cap. There will commonly be an arrow showing which way to twist the key, but not always, and it's not always clear what the arrow means. It might take moderate force, but don't apply brute strength, or you could snap off the key, leaving you well and truly stuck. Sometimes pressing down on the cap makes it easier to turn the key.
Once the cap is open (many bikes have a hinging cap, just fold it back to its stop), grab the nozzle, and put it perhaps an inch (2-3 cm) into the tank opening.
Make sure the metal nozzle is touching the metal rim on the tank filler opening! This grounds the motorcycle, preventing build-up of static electricity.
Gas nozzles are grounded, to prevent static build up as the gas is pumped. (Amazingly, gasoline moving through the pump's nozzle does build up static, sometimes quite a lot of it.) A static charge could cause a spark to jump when you pull the nozzle away, if the system wasn't grounded. You can probably imagine that a spark at the gas tank opening is among the world's worst ideas. At the very best, your bike only catches fire and burns uncontrollably. At worst, you get explosions like you normally only see in movies featuring guns and mean-faced guys in black turtlenecks.
If your gas nozzle has one of those pleated shrouds which allows it to catch gas fumes, you need to pull it back before the nozzle will work. Nozzles of that type contain an interlock device which will prevent them from pumping if the shroud isn't pushed back far enough. Since you'll never fill the tank with the nozzle shoved into the tank, you have to pull back the shroud by hand.
Start pumping with the nozzle an inch or so into the tank, until the tank is nearly full. You have to look into the tank for this operation to work, since the auto-off clicker on the nozzle won't work right to fill the motorcycle tank. Wear eye protection (such as your helmet or sunglasses) to keep gasoline from splashing into your eyes. If gasoline does get into your eyes, flush them with water immediately, then seek medical help.
There's commonly a visible lip inside the tank, and you can usually safely fill to that point. Don't try to fill beyond it unless you know what you're doing. If you fill the tank beyond that lip, the gasoline can expand out the vents and spill on the ground, creating a fire hazard, and wasting the gas for which you just paid good money. This is usually only a danger when parking the bike immediately after filling it. Note that on some bikes (particularly California models) you can destroy the emission system by filling the tank too full.
Once you've filled up the tank and closed the lid, check to make sure (if you have one) that your fuel petcock is set to the ON position, and not to RES or Reserve, or OFF. For more discussion on what reserve means, please see this FAQ article. If you have two petcocks, check both of them. Fuel injected bikes don't have a petcock at all for normal operation.
Most motorcycles don't include a fuel gauge. On those that do, the fuel gauge is usually inaccurate or worse. The best way to solve this problem is to use your trip meter. If your bike has several trip meters, pick one for gas and always use it that way.
Just reset the trip meter to zero before you leave the filling station. Note how many miles are on the trip odometer next time your bike hits reserve, or the fuel light comes on. Keep paying attention to that number as you use the bike, and you'll quickly get a feel for about how many miles until it's time to head to the gas station.
If you fill up to the same point each time, and record trip odometer miles and gallons filled, this will also allow you to calculate gas mileage. This information can be very useful in determining fuel range. I use Fuel Record, which is free software for the Palm Pilot, to track mileage, which makes it pretty painless. It's also easy to find a small notebook and record the numbers in there -- that notebook can be kept in your tankbag, or a jacket pocket.
At a minimum, you need to track mileage since the last fill (conveniently there on the trip odometer if you reset it with each fill) and gallons used. It's also handy to record the date and odometer reading; tracking which gas station/brand you use can also be helpful.