Overview

One of the first things to remember about the 18th century is that fabric is expensive, but labor is not.  If you aren't looking to portray the upper class, stick with linens, cottons, or wools.  My recommendation is to check with sutlers if you don't know what to look for.  Most places (like Wm Booth Draper and Burnely & Trowbridge) will gladly talk to you over phone or email about fabric choices.  If you are going high class, silk taffeta is your best bet.  Dupioni, while cheaper now, isn't correct.  The safest bets are solid colors, rather than prints.  There are many incorrect prints out there that take a trained eye to spot and there are some garments where prints aren't always correct.  Here is a basic list of garments for men and women.

Women

Shift: the basic linen undergarment which allowed for easy cleaning and kept outer clothing clean.

Stockings:  over-the-knee socks held up by garters or ribbons.  Came in many colors, some with decorative clocking.

Stays:  not a corset.  These gave you a conical shape without crushing you.  Half or fully boned, straps or no, front or front/back laced.

Pockets:  hanging pouches tied on around the waist, separate from the other garments, accessible through slits in the petticoat.

Side Hoops: sometimes called paniers in modern terms, give you the wide hips without the fuss of a large hoop petticoat.  Very popular in the '70s.  You could also use these as large pockets.  Replaced by bum and hip rolls in the '80s.

Petticoat:  pleated skirt that would be worn under the gown or jacket.  Under-petticoats were made of less expensive material and could add warmth or hide lines from hoops.  Usually about 3" off the ground.

Jacket:  there are many different styles of jacket.  These were most commonly worn by both working class and upper class, though in different styles.  Some fitted, some unfitted and held in by the apron.

Gown:  worn by every class.  A robe a l'Anglaise has a fitted back while a robe a la Francaise has a loose or "sack" back.  More money meant more trim.  All classes used ruffles or kerchiefs around the neck for modesty.

Cloak:  worn by women and men for warmth.  Made of wool and came with or without a hood.

Mantelet:  a shortened cloak usually made from satin, silk, or lace.  It was used for warmth, but often indoors or on cool days.
Robe a la Polonaise: a particular style of gown with gathered up skirts, loose fronts, and false waistcoat.  Many styles of gowns have polonaised skirts, but are not this specific style.
Brunswick: a sacque-back style jacket, often used for travel, that has a hood and removable long-sleeves.
Riding Habit: ensemble of petticoat, riding shirt, waistcoat, and masculine jacket.

Mens

Shirt:  similar to today's shirts, but with a front slit rather than fully open.  Always loosely fitted and often long enough to serve as underwear.

Waistcoat:  Buttoned vest worn by all classes.  Some have sleeves which could be sewn in or tied on.

Jacket:  Similar to waistcoat in cut and length.  Often had sleeves, but not always.  More practical than a full-length coat for work and often worn over a waistcoat.

Breeches:  Knee length pants that buttoned down to a buckled, buttoned, or tied cuff on the leg.  The top was also buttoned and usually had a fall-front.  The back was looser to allow for room to ride a horse.  It also laced closed in the back at the waistband to allow for adjustment.

Coat:  Knee-length, full-skirted (narrowing with time).  Worn over waistcoat by all classes.

Cloaks & Stockings:  generally the same as women's wear.

Shoes:  Men's are usually made of leather, exception in some dance pumps and slippers.  Women's could be fabric covered or leather (remember leather is the cheap option at this point).

There are a lot of items and terms I haven't covered, but this gives you the basics.