can someone pls tell me anything to learn for the letter
Learn the basic layout of the letter i.e. date, introduction, main body, conclusion, close. Know the basic phrases of letter writing- Querido/a, gracias por tu carta... , escribeme pronto etc. Other than that, there's not much you can learn off. Just increase your vocabulary and practice writing letters.
You should also revise the past, present and future tense. They usually give you something like 'Say 3 things you did last week' and 'Tell him what you will do on holidays'. Usually catches people out here. Hope this helps :)