You only need two letters (ä and ö plus the Swedish å) that are different from the English keyboard. But switching between keyboards may not be for you since you type most of the time in English and other things like :; @? etc. would also be in a different place on the Finnish keyboard. Instead do the following:
Mac: To get the umlaut above a letter: 1. Hold down the Option key, and type a u (the letter u). 2. Let go of the keys (don't hold them down for step 3). 3. Type the vowel over which you want the umlaut to appear.
Windows: Hold down the Alt key and type a code into the 10 key pad. Once you release the Alt key, the letter will appear. The codes are: Å is 143, å is 134, Ä is 142, ä is 132, Ö is 153, ö is 148.
There are many apps for learning Finnish. Simply search for Finnish in the App store (Apple). Here are some I have played with. Most of them are free to begin with. If you like the app you can purchase more content.
Learn Finnish Word Power
Finnish in a month HD
Mondly
Learn Finnish
Finnish FEL
Finnish by Nemo
50 languages
Kielitoimiston sanakirja (for inflections and conjugations)
Finnish Dictionary
You can watch Yle's subtitled programs in Finnish or Swedish. If there is a word that you cannot understand, you can click on the word to see a support translation in the language of your choice.
Web and mobile app. Free. Listen to hundreds of audio tracks from several Finnish books: From Start to Finnish, Suomen Mestari 1, 2 and 3, and Teach Yourself Finnish. Search by the booktitles. Great free resource.
A free and open source cross-platform multimedia player. Plays most multimedia files as well as DVDs, Audio CDs, VCDs, and various streaming protocols.
Finds the dictionary form for any inflected Finnish word.
Verbix. Conjugates any Finnish verb: Fill in the infinitive. Don't use any capital letters!
You can input 8094 verbs in any form, tense or mood in both Finnish and English.
Web and mobile app. Free. Search for a word in Finnish or English and Linguee provides thousands of professionally translated short paragraphs with the word in it.
It shows you a sentence missing the most difficult word, and the challenge is to fill in the correct word from context.
Oppi ja ilo. Very large collection and variety.