A Beginners Guide To Basic Melody

Using scales is ok, but it can kind of limit your melodic flow.

Several things I have worked out from analysis of folk tunes and pop songs over the last century (empirical or experimental evidence if you will - what others have done and so what you can do too to make memorable melodies.):

You will stick to the same note about 3/8 of the time

You will move up or down by step (1 or 2 black and/or white keys) about 1/4 of the time

You will move up or down by a 3rd (3 or 4 black and/or white keys) about 1/4 of the time

You will move up or down by larger intervals about 1/8 of the time

You can use the notes from the individual chords to start your melody and use the above principles to create your melodies, changing notes as you move chord to chord.

You can lay out all of your chord notes for any two bar passage and use those as your melodic source material. E.G. bar 1 is chord I so notes 1 3 5 are used, bar two is chord V, so notes 5 7 2 are also used; this means that over the two bars using notes 12357 for your melody is ok. Using chords I and bVII in a two bar phrase you would have the following notes available: 12345b7

You can lay out all of your chord notes for any four bar passage and use those as your melodic source material. E.G. bar 1 and 3 is chord I so notes 1 3 5 are used, bar two is chord V, so notes 5 7 2 are also used, bar 4 is chord IV, so notes 4 6 1; this means that over the two bars using notes 1234567 for your melody is ok - this is pretty every note within the scale.

The problem beginners face with melody writing is that their subconscious is judging it against other models of melodic design. Emotion in a melody is a combination of tones, rhythm and expressive techniques for the instrument playing the melody; i.e. the actual notes are part of the puzzle, as is the rhythm; the way that the notes and rhythm are played is the final part of the puzzle, so it won't make a difference if the notes and rhythm are great, it only becomes fantastic when they are played the right way with the right dynamics and articulations (legato (smoothly) or staccato (short, sharp), ff (very loud) or pp (very soft), tenuto (held) or rubato (literally robbed time - meaning that the rhythm is only partially stuck to and tempo is a guide rather than strictly observed)

Generally we look to falling intervals for sadness and rising intervals for happiness (an interval is the space or distance between notes for those who aren't sure)

The quickest way I learn to write melodies is to mimic my favorite artists and understand how they do it. 

Is their melody 4 or 8 bars? What key is the melody in? How does the melody interact with the rhythm section? What is the bass doing when the lead melody comes in? How are the chord changes modified in the transistions?

If you're new to this, unless you're some type of prodigy, you're not going to write a hit melody in the first shot, you're going to need to keep writing until it starts to make sense. Actively listen to your favorite songs and pick them apart. Collab with other musicians so you can get a feel for how they write. 

At the most simple level, think about it like this, there are only a handful of notes that make up any particular scale...it's how they are played that makes the biggest impact.