What I am doing*

Hi, how are you? I'm at home, I'm bored and I'm studying some English because I make a lot mistakes so I need to improve. I just finished** drinking my*** tea and I'm a little dizzy I don't know the cause, I guess it's because I can't breathe well. Today it's not very cold, yesterday it was colder.****

* Or: 'What I'm doing now'

** 'finished' for past tense
*** Because it is your tea, use the possessive 'my'

**** Here you can use the comparative of 'cold': 'colder'

You could also split your original sentence in two: 'Today it's not very cold. It was very cold yesterday, colder than today.'

I hope your dizziness passes!

What I'm (I am) doing

Hi! How are you ? I'm at home. I'm bored and I am (I'm) studying some English because I make a lot mistakes so I need to improve .I just finished drinking tea and I'm a little dizzy. I don't know the cause (or I don't know why) but I guess It's because I can't breathe well. Today it's not very cold however yesterday was much colder than today.