'Any old [something]' just means a more or less random example of that thing - nothing special or in particular. There's no dress code, so you can just wear any old thing. In your sentence, it means you should find something out about the company so that it looks like you're genuinely interested in working for them - not that you are just looking for any job. Do your homework so you give the impression you really want to work for them and you're not just looking for any old [more or less random] job.

here, "old" is just an affectionate word. a diminutive word. so "any old job" just means "any job".