You are not required to have a gi (white martial arts uniform) to start practicing aikido. When you are ready to purchase one we can help you with recommendations. Until that time you should wear loose-fitting clothing that covers your knees. Normal gym attire (sweats and a t-shirt) are fine.
In addition to the gi, some aikidoka choose to wear hakama (skirt-like pants). The hakama is a traditional piece of Japanese clothing while the standard gi, worn in aikido as well as in other martial arts such as Judo or Karate, originally functioned as underclothing. Wearing the hakama is more formal and is often required in more traditional arts like Kyudo and Kenjutsu.