are there prayers before mikva? what are the laws to go to mikva?

ב"ה Shalom It was not specified in the question whether you are referring to a male going to a mikve or a married woman going to a mikveh. The differences are great, but I will answer in short. For a man, going to a mikveh today, there is no real requirement. It is only for added piety and purity. Therefore, there are different customs of when men go to the mikveh, daily, before Shabbat, before Rosh Hashana and Yom Kippur, before the different holidays etc. Since there is no requirement for men, there is no Bracha said. For women, making themselves permitted to their husband after being Nidda, is a requirement and a Bracha is said and there are those who have a custom of saying a prayer, found in the Sefer "Lashon Chachamim" of the Ben ISh Chai (vol.II, section 23). The laws for a woman going to a mikveh require an entire book, which is not in the scope of this website. All the best