You probably won't be able to register for My Account because the CRA asks you to enter information from your most recent filed and assessed tax return to register. You can contact the CRA to double check what information they have on file for you.
For your first return as mentioned by @InnaE you will not be able to answer any taxes question related to your previous taxes return but you can create your account, so that next years all your data are stored in the CRA database.
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