We don't really say "we have started this job for eight weeks"
"I started this job eight weeks ago" is the way we would say it.
I suspect sentence 1 could look like this
"I have been working here for 8 weeks"
Here the only difference is that we are adding the "have been working". This sentence works well and means that you started 8 weeks ago and are still working on the job.
hope this helps and reach out if you have any more questions.