Generally, all direct-to-garment manufacturers will train you on the RIP software that is needed to run your printer, so don’t worry if RIP software intimidates you.
In regards to graphics – remember – you are a printer. Do not be afraid to give your customer guidelines and artwork specifications to your needs. Remind them that it is your job to print.
Usually, design work should be charged extra so if you wanted to offer that service, hire someone with those capabilities and use the extra charges to pay for their time and labor.