First, you need to clarify a bit what it is you want to build.
1. Do you want just light or do you want sound? Personally, I've built (and rebuilt) a few sabers, three with sound but my first was just light only to get me used to the idea of saber building and soldering again after more than 20 years.
2. Do you know how to or want to solder? If you don't want to solder, you can get the plug & play electronics. Saber Forge sells everything from Warrior tier (light only) to Hero Tier in Plug & Play, so soldering is completely unnecessary.
Once you figure that out, you need to decide what kind of hilt you want. Every hilt Saber Forge sells is available in the Initiate tier, for DIY purposes. That will get you just the hilt only. If you want something original, there's the ASP system. I have yet to experience building with a Saber Forge hilt, but I want to try both down the line. You can even use the ASP designer tool to virtually build a saber in 3D and see what parts you need to order.
After that, it's just the electronics. There are two categories for electronics, Plug & Play and DIY. Plug & Play is just what it sounds, it's prewired and you can easily install it in the saber. For that, you'll need to pick a tier, Warrior, Champion and Hero. Whichever one you pick, you'll need to add an LED module and switch. Latching for Warrior, and Momentary for Champion and Hero. Also, you'll need a battery and charger for the Warrior tier. (Champion and Hero come with both)
Also, don't forget to buy a blade.
That should be about everything you need. If you want to go DIY, be sure to say so, there's a considerably longer list of parts you'll need to get before you'll be ready to build a saber without prewired parts. I'm not suggesting you shy away from that at all, every saber I've built has been mostly wired by me. I just want to know how you want to proceed before I put together a much longer list of parts.