The fact that you are doing work for a company is a great start. All you have to do now is pretend you are an agency. Change your site so it appears like a company etc. I lied in the beginning and told clients there were 5 of us when it was two after meetings I always said I was off to see another company even if I was not. I had to keep notes on who I had told what encase I messed up.

It got us into the mentality and also our clients. To become an agency and to employ you need money and enough work to validate needing to get someone in to help.

Finding new work can be the hardest part because there is no formula. But basically you need to swamp yourself with work never say no and worry about how you will do it later this will force you to grow and become an agency.

Some points

win more business than you can physically complete deal with it by calling in a friend or working late.

you must look into the realistic monthly costs of employing and renting an office adobe licenses etc then work out how much your day rate would be and how much you would need to bring in a month to make a profit on that. Cashflow is soooooo important and will be scary when you see the numbers but something to aim for.

learn the game. How do agency's work? It's all about a company thinking you are An agency, put together standard terms and contracts when carrying out work these are easy to google for and adapt. Do things like 40% upfront on order payments and payment stages.

Hope some of that helps