Well asudhir, since i've been making sings for about 15 yrs. now (give or take), i feel i might be able to point you in the good dir.
First off, lightly sand the surface you wish to paint (400 grit sandppr or similar). Be carefull about the dust (dust gets everywhere)
Then with a damp cloth wipe the surface clean as best you can.
Use a cleaner that removes grease ie. windex or somth. (greasy finger prints can ruin a paint job)
For the type of paint i would recomend waterbased acrylics. (easy to clean and doesn't smell so offensive)
If you want to put the effort in to it i would suggest you use an acrylic primer first. (it really makes the paint stick better and makes it tougher and last longer)
Use brushes, tape an x-acto knife(to cut designs with) especially an airbrush if you have access to one
Check the labels on the paint containers to see the recommended surfaces (it's a good indication to how it will perform. Might stick for a while but if it's not the right stuff it wont last)
Hope i gave you some helpfull tips, if you got other quest. on painting feel free to ask.