Duolingo is for free and has finnish as an option now too
otherwise mondly is also a good app, but comes with a price.
then I know about the book series "Hyvin menee!" and "suomen mestari"
first one is praised by my classmates (i'm learning finnish at school) it seems better structured than suomen mestari, easier to follow, but suomen mestari is what is widely used in schools....so not sure which is better, but maybe you can borrow them to look yourself before committing?