The Delta-V (change in velocity) to reach the ISS (400km) in Low Earth Orbit (LEO) is 9.4km/s. This is the absolute minimum so it doesn't include things like drag or other inefficiencies. Getting to space takes only about 1.4km/s, just about all the rest of that delta-v is used to accelerate the spaceship to 7.6km/s. This is about the minimum speed, altitude, and delta-v to get into orbit and stay there. Going much lower than the ISS will have your orbit decay too much to stay in orbit for more than weeks or months. I think it would take about 8000m/s to orbit right at 100km but your orbit would decay really quickly at that altitude, you would only get a few hours of orbiting.

I think I answered you below about why you can't to orbit using only a railgun.

Edit: Going from earth orbit to other places doesn't take as much delta-v as you might think. Going from earth orbit to lunar orbit only takes about 3km/s and going from earth orbit to mars orbit takes 5.7km/s.