i have a 2000 b2500 se 147k on it. im not to mechanical.

when going around a corner, i hear a clanking sound.it sounds like the diff. also it has that wet look to it on the bottom like it was just in a puddle.
a friend told me i might have to do a new gasket and put grease in. not sure what that means.

 

It sounds like the diff, or third member is dry and need fluid. Check a manual for the correct weight and type, but it's likely 80w-90. Also, if you truck has a limited slip differential be sure to use a hypoid fluid or at least a hypoid additive to keep the limited slip functioning correctly.
If it's been dry for a while and with the noises you are hearing, you may need to have some parts/pieces replaced. A standard, or open rear end has side gears and spider gears that are activated when one wheel is asked to turn faster than the other, such as in a turning situation. It's likely you are hearing them screaming from a lack of lubrication. If a limited slip, then those side gears, or clutches do not have enough lubricant and are likely overheating/burning up.
Either way, and since you are not mechanically minded, have it checked out, and/or repaired by a qualified technician who can replace the bad parts and set up the final drive correctly for you.