If there is a melody, it should stand out. You can either do that by using one instrument in forte or louder, or combine several instruments to play the melody. For example, practically any combination of strings and woodwind will do.
Most pieces have at least one theme, which gets repeated every now and then. It allows the listener to get familiar with your piece quickly. Be sure to include some variations though with every repetition, unlike in pop music, you should not do exact repetitions in orchestral pieces. Vary the melody, vary the texture, vary the tempo, … it’s a must.
Having one melody after the other or repeating the same theme over and over can sound confusing and/or boring. Allow some “idle-time” between the themes.