I've decided to + (verb)

'I've' is short for 'I have' and including the word 'decided' you are stating that you have made a decision or come to a conclusion.

Here are some examples:

"I've decided to accept the job."

"I've decided to complete my degree."

"I've decided to change my bad habits."

"I've decided to extend my membership at the gym."

"I've decided to form a chess club."

"I've decided to hand over my responsibilities."

"I've decided to help you move."

"I've decided to interview for the job."

"I've decided to increase my work load."

"I've decided to manage a store."

I was about to + (verb)

When stating 'I was about to' you are informing someone that you are going to be doing something at that moment or in the very near future.

Here are some examples:

"I was about to go out."

"I was about to go to dinner."

"I was about to go to bed."

"I was about to go to work."

"I was about to say the same thing."

"I was about to call you."

"I was about to send you an email."

"I was about to mow my grass."

"I was about to order us some drinks."

"I was about to watch television."