tToday I'm only going to write a little bit because I was in my son's neighborhood helping him with the groceries.

Do you mean that you can't write very much today because you have had such a busy day (helping your son)? "Today I am going to write a bit" is correct English, but it's ambiguous: "a bit" could be more than usual (if you don't usually write anything) or less than usual (if you usually have more time to write.) That's why I have added "a LITTLE bit."

A more natural way of saying this might be: "I only have time to write a little bit today because I spent the whole afternoon helping my son with the groceries."

It takes me all of the ook me the whole afternoon.

Or "the entire afternoon."

Now I need to leave it and go to do my exercise routine, but I don't want to go before toI write some phrases.

We say "need TO" + verb
but
"before" + person + verb (before you go, before you write)
Or "before" + verbing (before writing, before leaving)

Just a bBit

It's customary to title keywords in English titles.

tToday I going to write a bit because I was in my son's neighborhood helping him with the groceries.

It takes me all of the ook me the entire afternoon.

Now I need leave it and go to do my exercise routine, but I don't want to go before to write some phrases.

Hey there.

you cant include this as a standalone sentence.

Hey, so today I was going to write a blit because I was in my son's neighborhood helping himtle, but I had to help my son with the groceries.

It takes me all of theup the entirety of noon.

Now I need leave it'm forced to skip writing and go to do my exercise routine but I don't want to go before to write some phrases.give up that easily. I need to write some sentences before I go.

Ok, that i's all for today.