What are all the components used in this sketch?
My advice is not to start with this, or at least minaturise it - 70A is a huge amount of current to be pulsing through a coil, and some pretty nasty things could happen if you get things wrong (burns, explosions, electrocution, etc)  

If you really want to make it, I'd start with the same circuit, but make the coil and current smaller (use a small bank of AA cells or something, keep the current under 5A or so) It will still work with a small coil, just be less powerful but much safer, and you will still learn as much whilst large designing it.

You need a (reasonably hefty - check the maximum Id specs in the datasheet) N-ch MOSFET for the coil switch, and an optocoupler as Gustavo has mentioned.
The reverse biased diode across the coil as Wouter mentions is a very good idea, certainly if it's your first project of this type (otherwise seriously high voltages can appear at the drain of the MOSFET - we are talking possibly in the region of a few kV, so please be careful and start with low currents)     

A resistor in series with the diode will actually increase the current discharge speed, but you need to calculate it carefully (it depends on the current switched, how fast, and the coil and MOSFET specs). This coilgun related page discusses switching an inductive load. If you are not sure about the resistor value, don't include it. 

Do a lot of reading before you start out, don't rush things, be safe, and have fun ;-)