You need a straight stick, a weight (rock), a long piece of string or duct tape, a glue gun if you are bad with knots, and you can add a kitchen knife to it.
Step 2: Adding the weight
To add the weight, place the rock near the tip of the stick, (I would say place the weight 5-6 inches away from the tip of the stick) rap some duct tape around the rock, (enough to stick the rock there for good) or rap some string around the rock a few times tie it and glue gun the areas where you placed the string both of the ways should make the weight hold it's place very well ( the pic may be a bit blurry)
Step 3: Adding the sharp tip
This step is really easy just stick a knife on the end of the stick near rock with duct tape. Or sharpen the end.(for pics look at pic for intro and pic for step 1)
Step 4: Throwing the javelin
hold javelin in your fist over your head throw like you would throw a really heavy dart.
WARNING:Do not throw at anyone or any animal (unless in self defence, then by all means, throw away)