The best place to exchange money in Yangon are Bogyoke Aung San Market a.k.a. Scott Market. The official exchange rate is quite far from black market rate. You should NOT better to exchange in airport booths or banks.

At the moment 100USD is about 780Kyat. 10,000JPY is about 105,000Kyat (They accepts JPY!)

Most of shops and restaurants and cheap hotels do not accept credit cards. USD is much acceptable. So you should prepare USD (clean and not folded) and the first place to visit in Yangon must be Bogyoke Aung San Market.