Provided that your lines flow well (voice leading), and have a nice contour, it's mostly creative decisions.
One thing to consider, is that many instruments have a "golden register", a pitch range where they sound best. On the other side of the coin, some tend to sound awful in certain ranges. The only way to learn this is through familiarizing yourself with the instruments you want to use, as well as studying orchestration in general. So, be careful of that.
A great "trick", is to "lead in" to the melody from the previous bar. Say that the first real note of my theme is an A, in A minor. I could lead in to that from the previous bar with two eighth notes of E and G below it.
A good example, would be this piece:
So, it starts with that famous Celesta line, which is occasionally harmonized with itself. As the theme comes back "home", we get those fast string runs and that oboe trill @ 43 seconds that imitates an owl hoot for a few bars, and then the horns play the theme.
Note the second theme, starting at 1:35. It's played by the winds, and then is restated at 1:48 by the trumpets.
You'll notice in each case, that there is just one note that leads in to restatement of the theme. In the first part, the horns play that fourth below the first beat of the bar where the theme really begins.
In the second example, it may sound like the trumpets just "start", but if you listen, the exact same thing is happening as in the first example. It's just that the woodwinds play that note (harmonized) before the trumpets begin; so you get the same effect.
The music theory term for this, is an "anacrusis"