Is Nocturnal Emission (Keri) a Sin?
A nocturnal emission is something that occurs by itself, against a person's will, and therefore the person is not considered to have committed a willful transgression and there is no punishment involved.
Nonetheless, t'shuva is certainly appropriate, especially if a person looked at things he shouldn't or had lustful thoughts during the day.
If you engage in heartfelt t'shuva over these mishaps, you can feel assured that Hashem accepts your remorse and forgives you each time. Don't brow-beat yourself, and reject feelings of failure and depression. This is an old strategy of the yetzer hara to sabotage your service of G-d. Adopt an attitude of happiness instead. It could very well be that these mishaps are occurring to inspire you to do more and more t'shuva so that you can completely atone for all the sins of your past. So you should feel happy rather than sad, that you have the opportunity to clean your slate completely. Try thinking that these occasions, and the feelings of t'shuva they cause, are getting you closer to G-d, not drawing you further away.
The holy Baal Shen Tov teaches that if a person has an emission without any cause or lustful thoughts, he should not worry because he was under a sentence of death for some other sin, and now, because of the great sorrow he feels in his heart over the wasting of semen, his broken heart takes the place of death, and he is absolved from the decree that was upon him.
1) Always guard your eyes carefully during the day.
2) Guard yourself from lustful thoughts.
3) The Kitzor Shulchan Aruch offers advice on preventing keri like reciting the first four Psalms before going to bed, and not overeating before retiring to sleep, and avoiding spicy foods..... The Arizal advises the concentrated recital of the Shema before going to bed as both a preventative measure and as a rectification of the souls that were taken captive by the waste of semen in the past. He also advises wearing a tallit katan while sleeping as further protection.
If you can immerse in a Mikva, it is praiseworthy, but it is not an Halachic obligation.
If the Mikva is hard for you, it would be good to at least wash yourself off in a shower. If you can keep the water pouring over your head for a few minutes straight, it can be considered like a Mikva in some ways.
But even without washing yourself off, you are allowed to daven and learn Torah since we pasken that אין דברי תורה מקבלין טומאה - "The words of Torah do not become impure".