Howdy.
I’m very much not in New England, but I can give you some ideas on what you need to get started.
At an absolute minimum, you need:
The instrument itself.
This needs to have the relevant removable parts (tuning pegs, tail piece), and should be skinned on at least the front side. A rear skinning is ideal for sound quality and volume, but you can play without.
Strings (ito)
You need at least one each of the first, second, and third strings. These should be the proper gauge for your preferred genre. Tell me more about your instrument and I can direct you. Back up strings are a must. As many beginners come to find, strings do not last overly long.
A bridge (koma)
This should match your preferred genre - but at all genres accept a spectrum of heights (and materials) that allow you to find something that matches your personal flair.
If you wish you play in a traditional style, you should invest in a bachi. In the beginning, don’t stress too much about material - just get something that is comfortable and not too expensive.
You would benefit from, but do not expressly need:
A (dou) gomu
This pad or sticker goes on the underside of the dou to stop it from sliding off your leg.
A yubisuri/yubikake
This cloth goes on your fingering hand to ease sliding.
A doukake
This paper, cloth, plastic, or lacquer guard goes on top of the instruments body, protecting it (somewhat) from sweat.
In addition, I would recommend:
A tuner
Shamisen tuning isn’t set to a specific key, but the relationship between the strings is important. Until your ear is used to it, a tuner helps (cell phone applications are fine).
A protective case
Shamisen are more durable than many give them credit for, but are top heavy and can break if you don’t store them properly. A case also helps manage humidity, which can cause warping, ruptures, and slips.
A washibukuro
This Japanese paper bag helps manage humidity.
A saofuki
Any soft cloth will do, but get something to wipe down the neck after use.
After this, some people like:
A fujaku (position marker)
This needs to match your shamisen’s scale length. Personally, I don’t think they’re expressly necessary. Adhoc stickers (with weak glue, which you replace frequently to avoid wood damage) on important positions are more than enough.