My 92 pajero keeps overheating after driving for approximate 10-20kms. I have filled radiator n reservoir and it is still boiling and using alot of coolant/water. Please help as I only just bought this car and am wondering if this problem will cost alot or can be easily fixed....

Pam are you sure it's boiling? If the Antifreeze is boiling I'd think you'd see steam coming out of your reservoir as the water expands into the reservoir, but you didn't say you see anything like that.
Of course engines get hot, that's why we have the cooling systems but it doesn't sound like it's "boiling" to me with no signs of visible steam unless your antifreeze level is really low?

If it's really boiling, and you antifreeze level is good, then your reservoir should be getting filled up with expanded hot water that goes into it to catch it when it expands.

If the water isn't going into the reservoir, then maybe your antifreeze level in the radiator is too low and you need to add antifreeze?
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Best to fill the radiator with the engine cooled down. If you ever have to open the radiator when it's hot, NEVER open it with your bare hand as you could get scolded with boiling water. Instead use a big thick towel to remove the radiator cap off a hot engine. Again, if you can, just wait until the engine cools then open the radiator cap.

With the engine cool, you can fill the radiator with a 50/50 antifreeze mix. The 50/50 mix come premixed 50 percent water, and 50 percent antifreeze. So you can throw that stuff straight into the radiator.

Non mixed antifreeze needs to be mixed 50/50. Some people use 50 percent distilled water when they mix it because distilled water doesn't have the minerals in it that may get stuck in your radiator. That's being a bit picky about it though. Tap water works fine in a pinch. The other 50 percent is antifreeze. You can mix it right in the radiator too, put in a measured amount of the antifreeze, followed by the same measured amount of water. The water pump will mix it all together when you run the engine.

Put the cap back on and run the engine and if/when the water "boils" it should go into your reservoir. If it does boil in go in the reservoir then yes...your engine is getting hot and the antifreeze is boiling.

When the antifreeze boils, it's probably because your thermostat is stuck closed. That means the thermostat isn't letting water circulate through the radiator to get cooled. Thermostats are cheap...no more than $30 I'd say for most cars. Changing them is pretty easy too as they are usually right at the top of the engine. A mechanic may charge $100 to change it....but it's an easy job on most vehicles requiring just a couple sockets/socket wrench.

You have a reservoir that has fluid level markings for hot and cold for the coolant fill it to the full mark for cold after the engine is cooled down. If this is the reservoir that burst and it leaks then replace it. If the engine just overheated and boiled the antifreeze out then find out why and fix that issue.

Yeah that is very possible the most common cause is because there isn't enough coolant in the radiator. If your radiator has a cap directly on it take it off and make sure the radiator is filled to the top of the neck with coolant. If the radiator does not you can remove the upper radiator hose and use a funnel to fill the radiator completely. If the temp gauge goes over 50% ever it's safe to assume you have a cooling issue. I hope getting your radiator properly filled with coolant fixes the issue. If not check for leaks in the water pump hoses etc. Check the thermostat.

try to replace the radiator cap w/ new ones,then do not fully fill the water reservoir w/ water leave some space for the air to circulate when water boils.done this and still there's a overheating your need to replace your cylinder head gasket,frequent overheating damages the gasket.

try to overhaul the radiator ,try to replace the radiator cap w/ new ones,try not to fill the water reservoir leave some space so when it boils air can easily rotate,if still doesn't work maybe u need to replace the cylinder gasket cause it also causes an overheating if it does have defect,or try to check water pump maybe its not working or has a problem

Thermostat?

I know it sounds redundant but, please replace your radiator reservoir cap;
they tend to wear out and, without extreme pressure that they provide to
modern engine systems the coolant boils. (you don't even see it leaving the coolant system as wispy steam as you drive. the physics are: more pressure higher boiling point.
this why modern cars can have much smaller radiators and, use auxilliary electric sensor
controlled fans.

head gasket is blown

From your description of symtoms it sounds like you have a bad water pump. If the coolant is not circulating properly you will have a heating problem. Sometimes even a new thermostat can stick and cause the engine to overheat. I have seen them fail in as little as a week after they were installed.