When you making advertising , what the first thing come in your mind ?

The message I want to communicate, the medium through which the ad will be delivered, and the target audience.

Target Audience and the Message I want to deliver.

Brand Awareness and Return on Investment

First thing to think about is how to make an ad that will grab the attention of the targeted audience and deliver the message intended as clearly and quickly as possible.
Also to make it as memorable as possible.

What is the need of the target and wich medium will be the best to interact with them.

it's ROI

What's your brand and who's it talking to? Followed by a few dozen questions, thoughts and scribbles that might lead to a creative solution (if not, start all over again till you hit the nail).
Don't give up.

The type of medium used, the product and the targeted audience (which are the main factors & the main limitations in almost any project)

Practicality

Advertising goal

The audiance you need to reach with your adverising plan.