You need to pick some specific pieces you need help with and post them here, if we are to help you.
In general, looking over your art, I think you need to work on color palettes (too many straight primary ramps) and there's a fair amount of banding, but silhouettes and construction seem fairly solid.
Then there's the issue of course, that Terraria itself has very mediocre to bad graphics and you're expected to match its style.