If you’re talking about receiving the sacrament of Reconciliation, you need a priest. None of the sacraments are do-it-yourself rites.
If you are talking about generally being sorry for your sins and wanting to ask for forgiveness, you can do that at any time. This section of the Catechism talks about doing penance for our sins: http://www.vatican.va/archive/ENG0015/__P4B.HTM
When we’ve committed serious, or mortal, sins, we need to go to confession. In other words, participate in the sacrament with a priest. Again from the Catechism:
1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God’s mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation: