First of all, when the engine is cold, make sure the coolant level is correct both in the radiator and the overflow tank. See if there's evidence of oil in the coolant (mud color), or coolant in the oil (milkshake color and consistancy). This indicates a head gasket or cracked head. If you have the means, you can pull the spark plugs and look for coolant residue on those. Also, do a visual inspection of the engine - look for leaking hoses, or external leaks on the side of the engine. Water pump, thermostat housing etc...
Head gasket problems on Nissans are very rare.
If not, start the engine and let if approach operating temperature - it will take a while just sitting there idling. When the needle on the gauge goes above normal, check and see if the electric cooling fans behind the radiator are going on. Your car may have one or two. If they don't come on at all and the car starts to overheat, there's your problem. Check the relay and fuse in the fuse box under the hood. Otherwise it's most likely a bad electric fan.
If the fans come on, and it still overheats, make note if it happens only when the car is stationary, or does it happen going down the highway. If it happens only under load going down the highway, it's possible that the radiator is blocked and a flush is in order. If it happens all the time (idling or driving), it's possible you have a bad thermostat.
Could also be a weak water pump, though I've never seen that on an Altima. You didn't say what kind of miles your car has...
Now, on to the common problems Nissans have.
There is a TSB for bad radiator caps on 99 and newer Nissans. NTB04-018. This can cause overheating and coolant loss. Inspect the cap - the rubber gasket on it should be the same size as the metal plunger behind it that it sits against. If it has swollen any larger that that, it wont vent properly and can cause a build up of pressure in the system and eventually blow by. If you're uncertain, just get another cap from the parts store - they're cheap.
There is also a TSB for refilling coolant in Nissans. NTB02-011a. It's difficult to refill the coolant properly if a significant amount has been lost. The problem is you can't get it filled all the way without ending up with an air bubble in the top of the system. There is a special tool used to refill these systems what works with shop air. It created a vacuum inside the cooling system that literally sucks coolant out of your container into the engine.
Next, on to cooling system basics.
Antifreeze works better under pressure. When the pressure goes up, the boiling point also goes up. If there is any kind of leak in your system, perhaps a hose, or water pump or radiator cap, the pressure goes down in the system and the operating temperature will go up - sometimes to boiling level.
Also, when a head gasket goes, one of two things can happen. Combustion gases, or engine oil can literally be pumped into the cooling system, as those passages are very close to each other through the head gasket. We use a "block tester", which is a way of sampling the gases coming out of a hot radiator and testing it for the presence of combustion gases - it's simply a die that changes color. Also, some shops can do it with an emmissions tester - the same probe that is stuck into the tail pipe can be held above the opening of the radiator to test for those gases. Another test we do is called a pressure test, which places the cooling system under increased pressure. A pressure drop is noted on a gauge, and we know to look for a leak. If it's not external, most likely it's a head gasket.