A good place to start is thinking of melodies that you enjoy and figuring out why you enjoy them. Is it a certain energy that it possesses, is it the way that the melody interacts with the harmony, or is it something else? Understanding the music you enjoy can help you to understand your own tastes and, in turn, help develop your own voice. This is probably the most common place that I begin when I want to write an original melody. Never be afraid to look to someone else's work for inspiration.
Another way is to simply start singing--out loud, or in your head--and just see where your voice takes you. Often times I'll find that my best melodies are conceived this way. It's a very genuine way to compose, simply because there is no immediate filter. What is in your head, good or bad, comes out. It's then up to you if you want it to be written down.
Some melodies are excellent on their own, but sometimes a melody needs support from the harmony, a counter melody, a rhythmic motif, etc. Don't be discouraged if what you've written begins to feel uninteresting over time or too simple (or even overly complicated). Perhaps it just needs a change of context. Instead of changing the melody itself, try reharmonizing it or changing where the melody begins and ends in the measure. The results are often unexpectedly refreshing.
Hope this helps!