Use a compressor with a holding tank and I suggest having a backup airbrush compressor, just in case there are problems with your primary compressor. I prefer industrial pancake compressors. I only use a silent compressor if noise is a significant factor in the location that I am airbrushing.
The two most popular airbrushes are the Iwata Eclipse and the Paasche VL-3. Both are great brushes for airbrushing shirts. The Iwata has a bit finer line but is parts are a bit more sensitive than the VL. Keep your airbrush clean and keep spare parts in stock, especially tips and needles. Learning to keep your airbrush clean and functioning is just as important as your airbrushing skills.For most applications including airbrush t-shirts, clothing, illustration, and license plates the two most popular brands are Createx and Aqua Flow. Both have bright colors with good wash fastness. I personally prefer Aqua Flow because it is a bit thicker than Createx. Createx takes a bit longer to dry, and I have to be careful not to spider when painting on solid objects.
It saddens me when I hear of people that have purchased an airbrush t-shirt and left with the instructions to iron their shirt before they wash it. I do realize that there are some instances where an airbrush setup cannot support a heat press, however, I believe in providing a complete and finished product. Customer can purchase and wear or give as a gift, and not have to worry about the shirt fading. A quality heat press will cost more than an industrial compressor and several airbrushes, but it is a worthwhile investment.
Use top quality name brands of 100% cotton or 50/50 t-shirts. Some top brands are Gildan, Anvil, Jerzees, Hanes, and Fruit of the Loom. 6.1 ounce is considered heavyweight. Always keep plenty of stock. Kid sizes through Adult 3XL, or 4XL. I stock acmany colors now, but initially it was a lot of White, some Ash and some Black. When you learn how to airbrush, speed will come with time. If you are just starting out, take your time. Work on perfecting the basic skills and the basic movements. Focus on producing a high quality product.
The key to any business success is creating and keeping customers. To do this you want to be courteous and professional. Work to educate your customers about what you do. Listen to your customers and find out exactly what they want, and then give them what they want. Work to keep your customers happy. If you make a mistake, fix it, even if it means doing something over. If a customer is unhappy, do what you can to make them satisfied.
Be creative. Practice and strive to improve. Improve your airbrush lettering, improve your airbrush designs, and improve your selling techniques. Purchase instructional airbrush DVD's that interests you and seeks out videos on websites such as youtube to expand and improve your skills.