How to shoot in the direction the player is looking at?

For the x/y/z velocity of the projectile, use the 'x/y/z look-angle vectors' of the player. (There's a seperate x-look angle vector, z-look angle vector, and y look angle vector function.) These essentially get the individual components of the vector, (the arrow the player's looking at), which conveniently can be turned into velocities for your projectile to aim it as you desire.

If you want to increase or decrease the speed of the projectile, multiply each of the vectors, either by a decimal number, (for a slower speed), or a whole number, (for a greater speed.)