Okay, then all you need to do is to first type a quotation mark ("), then type the letter that needs an umlaut.
" + a = ä
" + o = ö
" + u = ü
It's extremely handy.
I think another letter you'll need it for in Swedish is å.
To get this, you just need to hold alt and press the W key.
In addition to what LsoYtodysm said, you can use
right ALT + q for ä
right ALT + w for å
right ALT + p for ö
right ALT + l for ø
Add SHIFT for Ä,Å,Ö,Ø. I'm not sure if there will be any other letters you might need for Swedish, but right ALT + z for æ is useful in Danish, too.
Ni can technically refer to the formal singular you in Swedish (like Sie in German or vous in French), but in practice it has fallen almost entirely out of use and is archaic to the point of being perceived as weird or even rude.
Du always means the singular you.
Ni in practice always means the plural you.
The letter a in Swedish is pronounced /ɑː/ long and /a/ short.
The letter ä in Swedish is pronounced /ɛː/ long and /ɛ/ short.
In other words, a is like the vowel in thought or car. ä is like the vowel in bet or then.
You can usually tell if a vowel is long or short by how many consonants are in front of it: long vowels will have one consonant, short ones will have two. Thus:
en mat (a food)
matt (listless, dull)
en häl (a heel)
en häll (a flat rock)