There are two main tests that are internationally approved; TOEFL and IELTS.
Check with your University/College how much they want you to achieve from each section (writing, reading, speaking & listening) and then...start practising! I made the mistake of googling some discussion pages and reading some comments saying these tests are an easy piece of cake, but that is not the case! Even if you have successfully passed Finnish 'Ylioppilastutkinto', especially the speaking part and time limit are things that WILL make u struggle unless you practice.
However, DO NOT buy those packets that send you online studying material for a really expensive price, because you can only usually do them once, and for instance my computer stopped working while doing mine and it ended up pointless. There is free online study material which should be perfectly fine.
Oh, and finally, the 'top' universities usually expect higher scores than the average ones. For instance, in TOEFL you can get maximum of 120 points, of which Oxford wants 100, but my university (Westminster) only needed 85. More news and information about university ranking here.
Love, and good luck with applying! xx