You can begin simply by telling the priest that it has been a long time and that you need help. Prepare honestly, ask God for mercy and be willing to take practical steps away from sin.
A little more context
An examination of conscience helps you reflect before you arrive. Confess serious sins you remember after a reasonable examination; do not deliberately conceal them. You do not need an exhaustive account of everyone else’s actions. If you are unsure how to describe something, ask the priest.
A practical beginning
Consider an appointment if you expect to need more time. Bring a brief paper note if it helps, keep it private, and dispose of it appropriately afterward. Repeated anxiety about already-confessed sins is something to discuss with a consistent confessor.
Source & further reading
Catechism 1450–1460: preparation and the acts of the penitent (opens in a new tab)↗Teaching summaries are introductions. Follow the linked Church source for its fuller context.