"I can't focus on what you told me to."
You don't need to focus to meditate.
Simply hold your head up and observe the barrage and motions of feelings and thoughts.
Now you are sitting
This is good.
"I forget to observe and go off on a thought."
That is fine.
Once you notice you've gone off on a thought or feeling,
Go back to observing them.
"Sometimes I want to act on my thought."
Then you can either act on it later and sit and observe it further, write it down, or you can act on it.
Then come back to sitting.