Most people use the software that comes with their router - for starters.
Fabricating acrylic letters is a huge learning curve, not simply a press-button job.
If you've never used a router before, I suggest you set your sights on simpler things, until you get some experience with your own machine, and your own software. You can't buy a bicycle and expect to be able to drive a truck in the same day...
We use Corel + e-cut + whatever else it takes to do the job -hammer, nails, heat gun, glue, hand-held router, laser... Every job is different.
Enjoy the learning curve!