Summarize:

Go to https://www.roblox.com/games in your browser. It's on the upper-left side of the page. If this is your first time opening the Create tab, click the blue Continue to Create page link in the pop-up window. Skip this step if you go straight to the Create page. If you aren't logged into Roblox, you'll need to enter your username and password and then click Sign In before you can continue. This option is near the bottom of the "My Creations" list of items. You may first need to click My Creations at the top of the page to open this list. It's a grey button near the top of the "Create a Shirt" page. A window will open. Find the PNG template image in the folder where you saved it (e.g., Desktop) and click it. It's in the bottom-right corner of the window. In the "Shirt Name" text field, type in the name of your shirt. This is how it will appear in the web store and on your profile. It's a green button below the "Shirt Name" text field. Doing so will upload your shirt to your Roblox profile, from which point you can equip or sell it as you please.
Open the Roblox main page. Click the Create tab. Click Continue to Create page if prompted. Click Shirts. Click Browse. Select your shirt image. Click Open. Enter a name for your shirt. Click Upload.