The Compendium of the Catechism says the following:
Sin is “a word, an act, or a desire contrary to the eternal Law” (Saint Augustine). It is an offense against God in disobedience to his love. It wounds human nature and injures human solidarity. Christ in his passion fully revealed the seriousness of sin and overcame it with his mercy.
(CCCC. 392) ( found here: vatican.va/archive/compendium_ccc/documents/archive_2005_compendium-ccc_en.html#The%20Dignity%20of%20the%20Human%20Person)
Sins can be both sins of commission, in which you choose to do something, and omission, in which you choose not to do something that you’re supposed to. If you didn’t do something, say you saw somebody have a heart attack and didn’t call an ambulance, then, yes, you’d have to confess that. Stealing, for example, could count as “not respecting private property”, which is technically an omission, but stealing is an active thing and not a passive thing, and, therefore, more accurately referred to as a commission.
You don’t have to tell the priest “I did X on Friday at 5:30”, if that’s what you mean. In the beginning of the Sacrament, you tell the priest how long it’s been since your last confession, and this suffices.
Here’s the full article from the Catechism.
Also, here is a helpful article on Reconciliation: ewtn.com/library/BISHOPS/GUIDEPEN.HTM
By the way, I am happy to see that you are interested in Catholicism! You are in my prayers.