If aliens from another star system have ever been to Earth, then there are 3 possibilities (that I can think of) of how they got here:
1) Their star system is close to ours, and they traveled here over many years at sublight speeds.
2) They found a way around the speed of light (used a wormhole to travel around the space/time continuum).
3) They found a way to travel faster than light while avoiding the infinite energy expenditure needed to reach the speed of light stated in Einstein's theory of relativity.
The easiest would be option 1, obviously. But, it's also the slowest method. #3, to me, seems the most difficult, because it goes against everything we know of regarding relativity and physics.
But, with an advanced enough technology...
"Any sufficiently advanced technology is indistinguishable from magic." -- Arthur C. Clarke