Welcome, you of no name and few years:)

>Hello

Greetings are covered somewhat in lesson 2 and somewhere
in the archives too but I'll just reiterate. The most useful greeting follows.

heill - to a man
heil - to a woman
heilir - to a group of men
heilar - to a group of women
heil - to a group containing both sexes

>Goodbye

Hmm... The word above is all-purpose but if you want to be specific
you can add 'kom(ið)' when greeting and 'far(ið)' when saying goodbye.

Kom heill/heil! - Come whole!
Ver heill/heil! - Be whole!
Far heill/heil! - Go whole

Komið heilir/heilar/heil!
Verið heilir/heilar/heil! (the plurals)
Farið heilir/heilar/heil!

>Thank you

I don't know what's the best way to say it; in the sagas indirect speech
is almost always use "hann þakkaði honum" = "he thanked him" but I
suppose at least some of those would be valid:

Þökk.
Þökk fyrir.
Þakka þér/ykkr/yðr. (singular, dual, plural)
Þakka þér/ykkr/yðr fyrir.
Haf(ið) þökk.
Haf(ið) þökk fyrir.

>Yes
>No

Not much used. Usually you just say "Svá er." = "Such is."
(or something similar) to express consent.

yes = já
no = nei

But if you use those much it looks like modern speech!

Regards,
Haukur