You need to learn to play legato without relying on the pedal. Try finding the most legato solo piano piece you can find... and play it through without touching the pedal. Keep doing it until you can make a convincing legato effect without using the pedal. This way, when you play string parts... you won't get the hideous overlap that occurs when you try using the pedal.
Another thing to remember is that an orchestra has lots of different instruments playing lots of different melodies. Try to emulate this on the keyboard. Don't just think of all the notes you're playing as one melody. Think of them as a couple different melody lines... each with it's own character. If you want, you could try to figure out maybe one line for your left hand and one or two for your right.... then practice each one individually.
Hope this helps...