His first symphony is a student work of sorts, and in honesty, it rather feels like a student work. He makes use of Russian folk melodies throughout, but his orchestration is right out of the German tradition, and the composer even admitted heavily relying on Berlioz's Treatise on Orchestration (one of the classic texts on the subject, to this day) and the advice of his teacher, Mily Balakirev. Of course, Rimsky-Korsakov himself would mature into one of the greatest orchestral colorists of all time, but that was still in his future.
Here is the Symphony No. 1 in E minor, by Rimsky-Korsakov.