There will be trial and error when it comes to the general learning phases of getting started.
When it comes to actual design requirements requested by various textile manufacturers you should just contact them and ask. Look at sites like alibaba, thomasnet, etc to find producers of goods. Some manufacturers don't mind if you send them dimensions on a napkin, they will just charge you for the design to CAD service. While others will required a extremely specific formatted CAD file. So you will just have to poke around.
Who is going to design your items? If it is you, do you have dozens of ideas ready to go? A few ideas will get you started but it won't take you far.