Actually: Fyrir (einni) viku síðan.
Prepositions commonly used with adverbial phrases of time:
fyrir+dat = _______ ago
fyrir ári -a year ago
fyrir tveimur dögum -two days ago
Fyrir viku is the most normal / natural translation of "one week ago".
The word einni isn't needed and síðan is a danish loan-word used in Icelandic and (*doesn't sound good).