For a brief discussion about inadvertant nocturnal emissions (wet dreams) and their ramifications, see this article.
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.
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After Keri
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".