First of all, this topic should have been placed in another forum - and most probably it will be moved by some moderator later on...
"Det sista jag behöver" reads: (I am underlining the stressed syllables)
Det´=> de (with "e" as in "bed"). Final "t" is silent. sista => sista (with "i" as in "sick" and "a" as in "duck"). All consonants read like in English.
jag => ya (with open "a", as in "car"). Final "g" is silent.
behöver => behöver (first "e" as in "bed", second "e" as "e" in "sister". ö does not exist in English, but it pronounced as it is in German; or the French "soeur")
på => po (closed "o", as in "mortal").