Using a rail gun depends on how good you are with Fixed aiming as there are no gimballed variants... if you're good and you hit a majority of the time it can be worth using. However, if you're not necessarily a good shot with fixed weapons you'll end up using all your ammunition and a lot of time hitting VERY few times, in which time you'll find you've not really done a lot of damage.
For a sidewinder I would simply recommend getting 2 gimballed pulse lasers and heading out to a Nav beacon. Farm wanted enemies there until you can afford a Viper (it really wont take long) or a Cobra
If you aim for something along this lines of this:
http://coriolis.io/outfit/viper/03D3D3D2D3C3D2C27271818--0243--.Iw1+kA==.Aw1+kA==
As your most basic starter killing vessel you won't go far wrong (obviously you'll then want to upgrade things like your power distribution system as the No.1 priority) and then you can work torwards upgrading all those modules to A-class variants. I find MC's and pulse lasers are often the most effective until you can get into a Vulture... then i changed to Cannons since the large pulse laser can melt smaller ships without need for a kinetic weapon.