Prepare for Yukata
First of all, prepare what is needed. You can find a 'Yukata dressing set' including underwear, belts, date-obi, etc., which costs about 3000yen. If you do not have anything but only Yukata, this set may help you to save your money.
The Price of the basic set
There are variety of basic sets raging from cheap to expensive though, it is not necessary to invest too much money on it as it is only underwear after all. It is enough if it includes underwear for Yukata, two waist strings, a Korin-belt that is a rubber string with clips on both ends, and a Date-jime.
Originally Yukata was put on the bare skin. Nowadays, we should put underwear to dress up and prevent sweat from staining the Yukata. For kimonos, the underwear is overlap style. You can also use ordinal camisole or chemise as long as it doesn't show from the collars.
However it may be safe to use underwear for Yukata as white or light color Yukatas are coming up this year. It is as sheer as it seems not.
You don't use a bra but a sport bra instead. Big breasts should be suppressed as much as possible in order to maintain the style of overlapping collars in front.
They used to use Sarashi, bleached cotton, to wrap the breasts in the authentic manner.
Koshihimo (a sash belt)
You need one Koshihimo in the meantime, two Koshihimos if possible. Wide type of Koshihimo is recommendable as narrow one bites into your body to make you suffer.
A Korin-belt is a rubber belt with clips at both ends. Also called one touch belt. It pulls both ends of the collar to maintain the form of a kimono.
There are variety of Date-jimes such as an old-fashioned obi type used for a Yukata in Japanese-style hotel, a rubber belt attached with Velcro, a mesh type for summer, fabric in front and rubber on the sides to tie with strings, etc.
Whatever the case, it is easy to use elastic one.
But a Date-jime made of only rubber may be hot.
A stretch Date-jime can be used for kimono as well.
* You can use a towel or a bra for kimono to control the style. But you don't have to use them. It makes you hotter after all.