If you have a martial arts Gi (white heavy cotton uniform as worn in Judo) then this would be ideal for a first lesson.   

 

If you don't have a Gi don't worry, loose clothing, track suit bottoms and a t-shirt will be fine. Be prepared to sweat! 

 

Before you go out and buy a new Gi it's probably best to try a few lessons and speak to someone at the Dojo about the best options. 

 

http://www.ninecircles.co.uk/ is a great supplier of martial arts gear and supply a specially designed Aikido Gi.