CONSTRUCTING A MELODY:
The easiest and most natural way to do this is to sing or hum whatever sounds appealing. It is important when constructing a melody to a chord sequence to experiment with different melodies until you find the absolute best one you can for your own personal taste. Through a little patience and experimentation it is usually fairly straight forward to come up with little vocal hooks and/or a 3 or 4 note melodic cell. These can usually be strung together into a sequence or words can be found to suit the melody which may indicate how it can be adapted to suit the nature of the words and subject material you are trying to communicate. It may be a good idea to sit at a piano or use an alternative instrument to construct a melody out of single notes either solo or to a chord sequence. This can then be translated vocally and again lyrics would typically follow. If constructing a melody to a lyric, it may be an idea to freeform sing the lyrics you have over the top of a sequence and experiment to see what you find. Anything and all ways are acceptable. Sometimes when free from singing a melody over a sequence can help to stumble on or suggest where the song needs to go for another section. Another potentially good idea to play with.
The melodic cell is the series of notes sound which the melody of a song is base. It is not uncommon for a vocal line to use around 3-5 notes and varying order. It often works really well and helps to keep an idea simple. Its a good thing to bear in mind though a songwriter does not have to stick to it exactly, the vocalist should have some freedom to improvise for the sake of once again keeping the listeners interested.
When constructing melodies for different parts in the song its important again to bare in mind the progression of the song in telling the story from start to finish. For example you could repeat the melody for the first verse in the second verse, but it may be a good idea to change the melody slightly as to defeat the expectation of the listener and again keep them more interested as to the potential ways the song may continue to defeat their expectation. So be interesting with you melodies, experiment and try different ideas, then see how many you can combine together in a way the works best for the song you are creating. It is not uncommon also to lift in the chorus where as to sit for a lowered vocal placement in the verse. but again these are just guidelines to be experimented with.
A hook in a song is a riff, lick, melodic phrase or catchy little snippet of something in a song that is catchy, nice and often hooks the listener into the song, hence the name.
USE OF EFFECT FOR PRODUCTION VALUE:
Sometimes if writing through recording on a computer or when using pedal boards etc. it is not uncommon to use effects to help to define a sound within a song and often to come up with interesting and/or unnatural hooks. For example if you put a bit of delay on your guitar, it instantly gives you more to play with, in terms of the sound of a part relative to how the delay is set. A bit like a vocalist having a bit of reverb during recording, just so their voice doesn't sound so dry.
Vocal hooks, outside from the normal vocal line in the song, might be repeated phrases or lyrics between verse and chorus etc, to help the song flow and keep the audience interested.
Harmony is the combining or more than one note that creates what is known as harmony. It is the relationship between the notes used and the affect in terms of sound feel of the combination that we call harmony, the combining of different frequencies of notes; Harmony can be consonant (something that sounds appealing to the human ear) and it can also be dissonant (uncomfortable to the human ear).
Harmony is technically more than one note playing at the same time, this can relate to vocal melodies and the harmonising of a vocal line as well as within each note in a chord, harmony applies to all of the notes used within a song. Vocal melodies are typically consonant, though it is more common to hear elements of dissonance with the make up of chord progressions and sequences.
Consonance and dissonance are defined by the relationships between certain frequencies or notes and they way the sound when played together. It relates to the frequency of each note. so say two frequencies were combined that had a mathematically positive relationship in terms of how many cycles per second, it would more than likely sound consonant. If two frequencies combined have a mathematically negative relationship it would sound dissonant, or the sound would clash more so to the human ear.
So if you had a constant frequency playing and a dial for another that you could turn and change the frequency up and down, you would find harmonic relationships as you cycled through. All of this is harmony, but it helps to have a good understanding of the way it works and therefore the way it can be used or the effect (flavour) it has in a song/chord etc.