If you come to Finland from outside the EU for more than 90 days, you must apply for a residence permit.
If you plan to work, you may need a residence permit even if your stay will be shorter than 90 days.
You need to apply for your residence permit personally. You cannot for example apply for a residence permit for your spouse or employee.
If you are a citizen of an EU Member State, Iceland, Liechtenstein, Norway or Switzerland, you do not need a residence permit for Finland but you must register your right of residence in Finland.
If your first residence permit is going to expire but you want to stay in Finland, apply for an extended permit or a permanent residence permit in good time.