What Should I Know Before My First BJJ Class?

Proper hygiene, etiquette, what to wear, and class procedures. Before going to your first class, you want to make sure that you practice good hygiene, follow your gym's etiquette (and BJJ etiquette), and know the procedures.

Hygiene

Trim your fingernails. Fingernails can get ripped, or you can cut someone. Always trim your fingernails before practicing BJJ.
Brush your teeth. You will be up close and personal with people. Nobody wants to smell your bad breath. Always brush your teeth before.
Wear deodorant. The same principle as above, you will be in close quarters. Nobody wants your body odor.
Be clean. Make sure that you are not dirty and if you are shower. You will not make friends if you are disgusting to your teammates.

Etiquette

Arrive early. Before your first class, you will most likely need to sign some paperwork and go through the gym etiquette and rules. Arrive fifteen minutes early so that you can go through all of this.
Introduce yourself. You want to make friends and make people feel comfortable around you. Introduce yourself to the lead instructor when you arrive. Once you are on the mats, introduce yourself to your fellow practitioners.
Treat everyone with respect. This is your first day you don't want to make a bad impression. Don't act like you know what you're doing (you don't). When sparring, don't be excessively rough, you will anger the people around you.
Follow the gym rules. Simple as that. The gym will have rules to make sure that you understand and follow them.

What to Wear

Athletic wear. You can change out of this and into your gi for a gi-specific class. For no-gi classes, you can wear your clothes for the class.
Flip flops. No shoes are allowed on the mat. Jiu-Jitsu is practices barefoot. Flip flops will make it easy to take your shoes on and off.

Class Procedure

Warm-Ups. Most classes will start with warm-ups. These are designed to get your blood moving, which will help you relax and pay attention to the instruction.
Instruction. Typically you will learn one or two techniques per class. You will then practice these with a partner. Practicing techniques should be done in a slow, controlled manner.
Sparring. After instruction, sparring will happen. The instructor may or may not allow you to spar on your first day. This depends on the gym and the individual.