I accidentally filed my 2017 return under the 2018 return section, how can i correct this mistake if it was already accepted by the IRS?

You cannot change or add anything on the return that you just e-filed.  It is too late, just like when you put an envelope in a US mailbox

Now you have to wait until the IRS either rejects or accepts your return. If your return is rejected, you will be able to go into your account and make the necessary changes to your tax return and re-submit your return with the right year info.

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund.  THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return.  Meanwhile, DO NOT change anything on your return until you start the amended return.

The 1040X form for 2018 hasn't been approved yet by IRS. Please, keep checking the following information on how to amend your return; it will be updated once the 2018 form is released:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

Then do you still need to file a 2017 return or amend one you did file?

An amended tax return for 2018 will need to be done to fix the errors.   Amended tax forms will be available on TurboTax on/around Feb 15th.