Every sport and discipline has their own equipment. Equipment is necessary for almost every sport, otherwise they cannot be practiced. Even running requires one to have a pair of running shoes, or any kind of cover, lest they want their feet punctured and cut by glass and all sorts of sharp objects.
Aikido is a martial arts discipline which does have its own equipment requirements. Some of it is mandatory, some customary, but all in all, it boils down to the following three things.
A Gi (Kimono)
Often called a Gi, one needs a uniform in order to practice almost every Eastern martial art. Aikido is not different in this regard, but, aikido does not have any specific gi requirements. Judo gis will work just fine, particularly if they are lightweight (judo being known for their throws and sudden movements, which require the gi to be tougher/thicker in order to withstand a single workout).
Aikido will have similar moves to judo, but nothing so tough that one will need a competitive judo gi. Gis are a standard in most Eastern martial arts so entering a dojo without one is very unlikely, unless one is there for a trial class.
Hakama
A hakama is a type of pants worn in traditional Chinese culture, but has since migrated to Japan (approximately in the 6th century). These pants are typically worn over a kimono, which would then explain its use in aikido. Hakamas are common in other Eastern martial arts, but not all aikido clubs/dojos place the same value on the hakama.
Check with your local club to see whether they have any special requirements regarding the hakama. Get a black or blue hakama, because these colors contrast the white gi.
Zori Sandals
Sandals are a traditional part of most dojos. They are traditional in Japan and China, overall, because it is impolite to get dirt on one’s floor, not to mention tatami in a dojo. Zori sandals are traditional slip-on sandals which are used to transport one from the changing room to the mats. It is important not to get one’s feet dirty and then dirty up the mats.
Zori sandals might be too much and most clubs will not bother if you have any slippers which you use exclusively for the dojo.
Bokken and Jo Staff
A bokken is a wooden replica of a katana sword. Practicing with bokkens is common in aikido, because one needs to know how to handle weapons, as well as arms and legs. A Jo staff is a staff, made out of red or white oak, traditionally, just like the bokken. Most dojos already have weapons purchased.
This is the essential aikido equipment any aikido practitioner might need.