Thoughts and opinions here only, from a former RCIA candidate…
If you’re not baptised then you won’t need to confess until after you are and any sin you may or may not have committed by way of failure to attend Mass would be cleansed at baptism.
However, if you are already baptised, consider your reason(s) for missing Masses. You aren’t committing a mortal sin due to your obligation not yet being ‘in place’ until reception…but are you sinning against the First Commandment if the reason falls more to the ‘can’t be bothered today’ side of the coin? If so then that would be an appropriate venial thing to confess at your first confession IMO (if you have time after the more serious stuff of the past).
You could still consider your current approach / attitude to missing Mass in light of the first commandment even if unbaptised of course.
Just for info, our RCIA calendar said we were ‘Expected’ to attend Sunday Mass.