Start something first. When you have earned enough over a year to live on, give your two weeks notice.

For me, I am aiming for 80% of my salary in a year's profit from my business before I leave the day job.

Never expect your job to educate you or expand your abilities. While it's nice when you have that opportunity, it isn't common and usually requires showing that you already are capable of doing the new stuff. It is not your employer's job to improve your skillset. You're hired for the skills you have and your ability to learn their practices.

Write an open letter to your boss telling him that you are not earning enough and want to quit to start something on your own. Publish it on as many outlets as possible, like medium.

He will understand and fire you on the spot, making it the easiest way to quit.

QED

Well if you're not going to go to another company or agency and are starting your own gig, here's some advice from someone who's done this a few times.

1 - Get your business plan together for your business. This will solidify what you're going to be doing and will give yourself and possible investors a road map for how to get there. I've seen so many businesses fail because they thought they had a good idea and just started throwing stuff together before really mapping out the financials, the product offering, the marketing and all that. This doesn't need to be 60 pages, but it does need to be fleshed out to some extent.

2 - If you don't already, put together a logo, marketing materials, business name, etc. These are the things you absolutely need to get awareness of your product out there.

3 - Depending on what country you're in, you'll need to form your business legally. That means you're going to have to register with the state (if in the US) and get legally recognized (incorporated) as a business. I would hire a business attorney to do it since it will save you time and money down the road. Also, having a legally recognized business will also absolve you from being sued personally if/when something goes bad with a client or company you're working with.

4 - Don't quit your job until the first three are completely nailed down and finished. You can waste months doing those and if you're not employed, you won't have the funds (unless you've already saved money in advance) to do some of these and being cash strapped while trying to get your business off the ground is no way to start a new company.

5 - Once you have the first three done, I would say its optional, but out aside some money so if things don't take off right away, you have some "float" reserves to keep you going so you can continue to work on your business. The last thing you want to do is to start something, have all your money drained and then have to give up and go back to work

6 - If you can, try and land some clients before you quit your current gig. This will give you a much better barometer for how good your product is and if people think its going to solve the problem you think it will. It also allows you some opportunity to generate some revenue so you're not starting "cold" with nothing at all.

7 - Once you have all those, then and only then would I consider quitting your current gig. Be polite, put in your two weeks and be affable till the last day and make sure you can use them as a reference. If there's one thing you absolutely never want to do, it's burn a bridge anywhere, for any reason. You never, ever know when someone, or some company may help you down the road.

8 - The last step is your first step towards freedom and being your own boss. Remember it takes twice as much effort, determination and courage to do something like this. So when you take that step out, have the courage to stick out to the bitter end, never give up and always be innovating and shipping.

Hope this helps, good luck!