It may seem like a mountain. But remember that a few years ago you knew absolutely nothing, and you have mastered a lot of material already! Three or four years is a lot of time, and almost certainly enough to become an expert on one thing (maybe even two). You can always expand later, but it's useful to keep your eyes open already. Attend seminars, organise learning groups, find peers with similar interests (and maybe some with different interests too!).
One major change going from undergrad (especially European undergrad, which is structured much more linearly than its North American counterpart) to a PhD programme is that you need to work on 'top down' learning instead of 'bottom up'. Try to understand the general ideas first before learning all the details. This takes time (just like the first year of undergrad took time to adjust), but it gets easier, and adjusting to this early will probably help a lot.
Attending seminars you don't understand is valuable, because you learn by osmosis. Someone will say something you don't understand, and you can go home and read about it in your own time. Or not, because you won't always be able to. But the next time it gets mentioned it will be less confusing, if only because you have seen it before.