I know that my answer below is more that what you have originally asked for. But, since you are just starting, I thought that the sharing of additional information would be beneficial to you.
One of the things that I do for a living is that I am a painting contractor. I am self-employed. Most painters that I know of charge by the square foot. If the paint is to be applied by a roller, around where I live, the price would be approximately .70 cents a square foot for two coats on a perfectly flat, non-porous surface with little or no imperfections that need to be repaired or patched. For textured and brick walls, I would make it .90 cents a square foot. If the surface has never been painted previously, or has any type of sheen to it, then it will need to be primed. I would charge .45 cents per square foot to apply one coat of a primer with a roller. I just include making small repairs and patching in the prices that I have already mentioned. If you do have to make extensive repairs to a surface, then I would make an estimate as far as how long I think that it would take me to do the patching or repairs for an estimate. For around where I live, I figure about $20.00 per hour.
You could call other painters in your area and just ask them what they charge per square foot for various types of surfaces.
I do like the above poster's recommendation about taking before and after pictures. Not only is this important for your protection, but a lot of people will be asking you if you have any pictures of work that you have already done.
As the poster above already mentioned about getting everything in writing, I agree that this is very important. You need to have a contract for a job that spells out, but is not limited to the following, as there are many sub-items in the contract that would be described under the categories below.
1. What the contract documents are. I.e. the contract itself and the painting estimate.
2. Description of work to be performed as described in the contract or in an attached work schedule.
3. Timing - when the work will start and when it will be completed.
4. Terms of payment.
5. Changes in work to be performed as mutually agreed upon by both the contractor and the customer.
6. Warranty for the completed job to be free from defects for a specified period given certain conditions are met.
7. Signs placed on the customer's property. Basically permission by the customer for the contractor to do that.
8. The customer's legal responsibilities.
9. Signatures and dates.
Other things to consider would be advertising, insurance, safety, the need for licensing, if any at all is required, bonding, etc.
You might like to check out answers that have been posted on the following links.
https://www.askmehelpdesk.com/music/...tml#post471334
https://www.askmehelpdesk.com/home-b...nse-76762.html
https://www.askmehelpdesk.com/home-b...ice-99405.html
https://www.askmehelpdesk.com/home-b...-il-93262.html
https://www.askmehelpdesk.com/home-b...ed-104538.html
https://www.askmehelpdesk.com/home-b...nse-86170.html
https://www.askmehelpdesk.com/home-b...ded-93189.html
I wish you well in your painting business!