The C value indicates how much current the battery can supply. Basically multiply the C value by the capacity to get the current.

A 22C 2100mAh battery can produce 22 * 2100 = 46200mA or 46.2A
A 16C 2100mAh battery can produce 16 * 2100 = 33600mA or 33.6A

You want the battery to be able to supply at least as much current as the ESC requires. If you're using a 35A or 40A ESC for example, you'll want the 22C battery over the 16C battery. If you're using a 30A or less ESC the 16C battery will work, 22C is more than you need but will work also.

To avoid overcharging a lipo, just use a good lipo charger and remove it when it's done charging (usually about 1 hour when charging at a 1C rate, which is recommended for most batteries). Most chargers will shut off automatically when full charged is reached but it's a good idea to not leave them unattended, don't leave them on the charger over night, don't leave the house while charging etc.

To avoid over-discharging, simply stop as soon as you start to notice the power dropping. Don't run it until the heli won't fly any more. Some people time a few flights to get an idea how long the battery will last then set a timer in the TX and land when the time is up.