Should I agree to referee a paper when I don't feel I can judge its importance?
I also started reviewing just recently (pure Math; mainly Algebra/Combinatorics) and when writing my first review with some assistance from my advisor, she taught me that it is not the reviewers job to judge if a paper fits well into a journal or not, that is the editors job.
Apart from checking the paper for correctness, giving helpful comments to make it nicer, etc. you can and should of course also judge it. However, I would not say "this is a good fit for journal XY because ..." but rather something like "this paper is really well written and generalizes the prominent result by A. and al." Or, if it is not well written, then you might also say that, e.g. "while the paper seems technically correct (up to minor points listed below), the page long calculations make it hard to follow and the authors fail to give an abstract overview of what they are doing."
The editor will then decide if the paper should be published in the journal or not, taking into account all reviews, maybe also how many papers were sent for this issue, etc.

Of course, as user2768 pointed out, you can let the editor know that while you will happily judge the correctness and readability of the paper, you are still to junior to properly give a statement on its expected impact.
That is totally ok and editors are aware that there isn't an infinite amount of perfect reviewers, who can comment on every aspect (in finite time...).
I would suggest that you also discuss this topic with your PhD-advisor; if he/she is willing to, you might even let him read over it before you send it.