Firstly I would let the fact that you're not coming "working in the IT sector" worry you. You'll bring a wealth of knowledge from the retail sector with you. See it as a benefit not a constraint.
You've done the right thing by choosing to do an online course, to me as a person who has interviewed DBA's that shows that
a) You're serious about getting what you want
b) You've recognised that you don't quite have the necessary skills but are trying to rectify that
c) You're prepared to do this off your own back and not being forced into it.
Get your certification, but in the meantime take a look at the entry level DBA jobs that are about. It wouldn't hurt to apply to a couple now, if you get an interview great! You may not get the first job you interview for, you may not get the second one either, but don't be dis-heartened. If you don't get a job, ask for feedback. Try and remember the questions you were asked that you didn't know the answers to. Research the answers so next time you'll get that job.
The key in interviews is to be confident, but honest. If you don't know, say you don't know. Don't try an BS your answer. The interviewer will see right through it.
Hope some of that helps,