Started with the 7mm starter kit, from PSI.
This included 2 each of Simline, Trimline, Designer NT, and Comfort Grip, in 2 finish colors each (typically gold and gunmetal or chrome).
This gave me a small variety to play with right away, along with the required bushings for each of them.
I enjoy turning the Comfort pens quite a bit ... nice thick barrels so it's easy to get a comfortable and pleasing shape and appearance.
You'll find what works best for you as you go, but the single barrel pen designs may be the easiest for a beginner...
The cheapest single barrel design pens I've seen lately, though, were about 5 dollars each from Smitty's ... the Le Roi Elegant Long Barrel (version 1 .... sell out 5-packs for 25 dollars in gold titanium nitride/black titanium nitride). The detailing and color of the finish on this pen is gorgeous, but the barrel thickness is pretty thin. You'll want something opaque unless you're OK with the barrel's brass color running interference with the colors of the acrylic blank you are using. Or you could backpaint the blank and/or the barrel before gluing it up. Possibly not the best choice for a beginner, for those reasons, but the price is more than reasonable and you can make these straight or add curves of your choice.