All universities that accept foreign students have an information service. Be it an International Office that is geared to meet the needs and questions of international applicants or at least an international website in English. You should definitively get in touch with these institutions to inquire what is needed for an application e.g. what are the required documents.
Usually the application process for international applicants at Finnish universities comprises the following:
Every university has an application form that can either be downloaded online or requested by letter.
Accompanying this form should be the following documents:
Keep in mind: All documents must be in English or Finnish. Translations of documents must be officially attested. And the grades must be visible on the certificates alongside with an overview of the country specific grading system.
Deadlines for the application can be looked up online on the Websites of the respective university.
International students are expected to have a secondary education that qualifies them to start a higher education program in their home countries. If you have an advanced technical degree you should contact the respective university to inquire whether it is accepted or not.
Even more often, however, it is the case that international students have already started a degree program in their home countries and want to do an exchange semester for a specific amount of time in Finland. Often times in these cases it is required that at least the foundation courses have been completed.
On the website of the universities or at the International Office you can also find out whether a degree program has any requirements concerning the grades you have to proof to have achieved in your secondary education certificate. The International Offices are institutions that can be found almost at every university and that is specifically set up to meet the needs and answer the questions of international students.
Another requirement that you have to fulfill is to proof that your English and/or Finnish language skills are sufficient. Whether you have to prove the one or the either is decided by the university and the study program. As this can vary from university to university you enquire the specific requirements at the International Office of the university that you are interested in.
To sum things up: In general it is advisable to contact the respective International Office as soon as possible to get to know the relevant deadlines and to request information material. Because admission requirements cannot only vary from university to university but also from year to year, they should be requested individually.