Have you managed to listen to it? It’s a lovely work but parts of it are not that easy. The first movement is straightforward hand-over-hand arpeggios (allegretto-ish) but they’re pretty pedally. Of the 2nd and 3rd movements, one of them is simple – just chords – and the other is moderately difficult if the conductor wants you to play those figures very fast; but you can make them easier for yourself by playing the ascending notes with one hand and the descending notes with the other, instead of doing it all with one hand as it’s written.
The glorious and dramatic fourth movement is pretty challenging, especially in view of the tempi – and bear in mind that you can’t hear everything that the harp has to do from a recording.
If you can get ahold of a CD or find it on YouTube, the best thing to do is give it a close examination following along with the score. You can download a PDF of it here:
http://ia600603.us.archive.org/34/items/gesngefrfrau00brah/gesngefrfrau00brah.pdf
If that doesn’t automatically load, try the main page and then click PDF in the sidebar to the left:
http://www.archive.org/details/gesngefrfrau00brah
Of course, a lot will depend on the speeds the conductor wants to take these at, so see if you can get him/her to give you metronome markings for the tempi.
Let us know how you get on! Best of luck –