Worm-fishing (or ice fishing in winter) does not require any permits. There are multiple worm-fishing rods (simple line and hook rod) at the cottage that can be freely used.
Fishing is free for under 18 year olds and pensioners. A fishing permit is required for 18-64 year old persons. The "Compulsory National Fisheries Management Fee" (16e per week or 47e per calendar year in 2024) allows lure-fishing in most finnish lakes and the fee/permit can be bought for example here (in english): https://kalastusluvat.kalapaikka.net/osta-kalastuslupa-e/
Some places, like fishing at rapids, can require an extra permit. They are often times locally available.