Is the grammar of this correct?
我的家


I Assume this is the title. If it is, perfectly fine.


  我叫Ian。我今年十四岁。


Too many subjects. Sound redundant. Better to be: 我叫Ian，今年十四岁。


  我的家在Greenlane。我的家有四口人，爸爸，妈妈，一个姐姐和我。


Redundancy problem again. Suggest to change to 我的家在Greenlane，家里有四口人——爸爸、妈妈、姐姐和我。（Note the correct usage of punctuation.）

I changed your second "我的家" into "家里" because


Although in Chinese, 家 can refer to either family or the house, please avoid refer to both in the same context, which leads to confusion.
We usually do not say "我的家有四口人". Instead, we say "我家有四口人" or "我家里有四口人"。In the second form, 我 can be omitted if your last sentence already uses 我 as subject.



  我的家有三个卧室，三个卫生间，一个厨房，客厅和一个饭厅。


This sentence does not have big problem in grammar. Maybe changing 我的家 into 我家 can may it more oral-like. Another thing to pay attention to is, you remove the quantity of 客厅 only, but keep all other ones with 一个. How about: 我家有客厅、饭厅、厨房、三个卧室和三个卫生间。. I also changed the order to make sure there's no ambiguity.


  我也有一个小花园。


也 is improper, since it implies other people also have a garden. If it's the only garden in your house, you should say 我还有一个花园. Another problem is, the garden does not belong to you only (in my point of view, it should belong to you family), so a better way to state is 我家还有一个花园. However, this would make the subject repeated. Therefore, you can change to another way around, saying 屋子外面还有一个花园.


  我的花园很漂亮。


我的 can be removed.


  我的客厅不大也不小。


Changing 我的 to 我家的, or simply removing it, would be better.


  客厅里有三个沙发，一个棕色的桌子，椅子和一个电视。


If you choose to remove 我的 in your last sentence, you can also remove 客厅 in this sentence and change 里 into 里面。Joining together the two sentences: 客厅不大也不小，里面有三个沙发、一个棕色的桌子、?把椅子和一个电视。（There's no reason to omit the quality of chair. If you omit it, some people may think there's only one chair. If you don't want to mention the number, you can also say 几把椅子.）


  我不喜欢我的客厅一点儿。


This one is too English. I assume you want to say: I dislike my living room a little bit. If this is the case, you should change into 我有点儿不喜欢我家的客厅.


  因为，在晚上，客厅很冷。


Everything fine.


  我喜欢我的卧室因为卧室很暖和。


Fine. But feel a little bit rigid/unnatural when moving from the last sentence. You can add some preposition, such as 相比之下。So the sentence becomes: 相比之下，我更喜欢我的卧室，因为卧室很暖和。. The second 卧室 can be swapped into 它 to remove repeating.


  还有，我的卧室比我姐姐的我是很大。


Typo? 我是 --> 卧室. Again, the second 卧室 can be removed. Another problem, 很大 can not be used to describe comparison. You can say 大很多 instead. The full sentence: 还有，我的卧室比我姐姐的大很多。.


  我的房间里有床，两个白色的书架，一个书桌和两个椅子。


Please don't change subject from 卧室 to 房间. This may makes the audience misleaded. Just say "卧室里有床、两个白色的书架、一个书桌和两把椅子。".


  书架上有英文书，韩文书和中文书。


Fine.


  我最喜欢我的家。


Better to be: 我最喜欢我的家了。 or 我非常喜欢我的家。. Because 最 makes people think you are making comparison.Either you like your home more than somebody else, or you like your home more than something else. Changing to "非常" would make it only mean the degree of your affection. 我最喜欢我的家了。 is another way of saying it with more emotion, instead of plainly state the fact.