If you do not have the correct training clothing for aikido (a white cotton jacket and trousers, called a keikogi in Japanese) you should wear loose, comfortable clothing such as sweat pants and T-shirt. Footwear should be used between the changing room and the dojo, long hair should be tied back and finger and toe nails should be short. Jewellery should be removed or covered with a plaster. Once inside the dojo, observe the etiquette used by senior students and follow their example.
The basic rules of etiquette include respect for your instructor, your fellow students and yourself. There is an emphasis on "rei" — bowing — not as a religious act but as a non-verbal means of showing respect. It is also part of your own training as some of the important aspects of Aikido training are contained within the practice of bowing.
If you are new to aikido you should speak to an instructor or a senior student for further basic etiquette.