Dear <here you write a name>
Thanks a lot for your last letter! I'm very happy that everything ok with your mom.
You were asking me how I'm going to spend my winter holiday break and to be honest I'm not sure yet. I was thinking about going somewhere, I would definately like to learn skiing but I'm afraid that I won't have enough money. I'll probably end up staying at home, watching TV and occasionaly going to the skate-rink. I'm sure if I'd told you about it but I'm quite fond of skating, not that good yet but doing my best!
Hey, maybe you'd like to come here during the break? We could go skating together, I'm sure you'd like it!
Looking forward to your next letter, make sure to write if you could come.
Yours <your name>
[Remember that a letter to a friend is an informal piece of writing, which means that you can use abbreciations like "I'm" or "you'd" which are not applicable to a formal letter like a letter of complaint for example.]