how to convert my 21 speed mountain bike into a road/cycling bike?
Start with your tyres. Go for slicks, the narrower the better. I used to run 1 1/4" tyres on my old mountain bike, at the time this was pretty much the lower limit on a 26" tyre. This will be by far the biggest bang for your buck. 

It will probably also blow your $70 budget if you get good tyres. You may also need to invest in new tubes, depending on what you have currently. But it will make a big difference.

The other thing that springs to mind is to improve your position. Difficult to do this much given the geometry of the frame, although I have heard of people putting aero bars onto their MTBs. Should improve things but its more cost. 

As for pedals - remember you'll need shoes as well - and these on their own will probably blow your budget. I'd stick with what you've got for now and think about upgrading as and when you have the cash.