melodic minor scales or chords

I need help,I'm really confuse.

I heard that C melodic minor scale is half major and half minor, or better it's neither major nor minor. and harmonically it shift back and forward between the major and minor.

if it's true, can some one explain to me how to use it....Thank you

You're right; the only difference from the C melodic minor scale to the C major scale is the minor third, E-flat, instead of the major third, E-natural. 

I think it's best to do this by learning the reason why the C melodic minor scale is the way it is: C-D-Eb-F-G-A(natural)-B(natural)-C. 

The reasoning behind the melodic minor scale is the desire to use the leading tone, B-natural, rather than the B-flat you'd find in the C natural minor scale. The leading tone makes cadences sound much stronger. 

So the B-flat moves up a half-step and becomes a B. At this point you have  C-D-Eb-F-G-Ab-B(natural)-C, which is known as the harmonic minor scale. It's used for the inner voices providing the harmony, hence the name. The A-flat is useful for harmonizing in lots of subdominant chords. 

But with the melody, the A-flat raises another issue: it's frowned upon to use the interval of an augmented second (three-half steps) in a melody. In the C harmonic minor scale, that would be A-flat to B-natural. 

Therefore, in the melodic voice, it's much smoother to simply raise that A-flat to an A-natural. This allows for easy upward stepwise movement in the melody: G-A-B-C provides much more "punch" than G-Ab-Bb-C, although you can find the second one all over today's popular music. It also avoids the awkward skip of A-flat to B-natural. 

So usually, you'd use the melodic minor scale for the melody and the harmonic minor scale for the backing chords. You'd just need to remember not to have any clashing tones of A-natural in the melody vs. A-flat in the harmony when you decide to use it. 

Obviously, this works the same way with all melodic minor scales, not just C! 

Sorry I'm so long-winded!

I don't dispute anything Noah said, but I'd like to point out that:

a. When going back down the scale in melodic minor, the notes are the same as the natural minor. Yup. So you go up the melodic minor scale in one way, and down another.
b. Since classical theory hates the minor v chord, melodic minor exists to give everyone the "normal" V chord that functional harmony just loves, though the v chord definitely has its own uses. 

Ah yeah, I forgot about the difference when descending! Good catch, @Monarcheon.

Well, that's the "rule" but even Bach didn't follow it sometimes. Yes, surely those G# and F# can be seen as passing non chord tones, but he could have followed the descending scale as it was supposed, but he didn't:

18 minutes ago, Luis Hernández said:

Well, that's the "rule" but even Bach didn't follow it sometimes. Yes, surely those G# and F# can be seen as passing non chord tones, but he could have followed the descending scale as it was supposed, but he didn't:

 

 

Then it's arguably not a melodic minor scale. It's a different mode of some sort.

I have not studied fugues that much, but I remember it written in "The Study Of Counterpoint" 

https://www.amazon.com/Study-Counterpoint-Johann-Joseph-Parnassum/dp/0393002772/ref=pd_lpo_sbs_14_t_0?_encoding=UTF8&psc=1&refRID=8TK456S0DH95J8R2ZPP0

where a melodic minor statement that descends normally with the flat 6 and 7, will need to be modified to harmonize correctly with another voice which is ascending, therefore using the natural 6 and 7. In these cases you would descend the same way you came up, to avoid nasty harmonic clashes, and to defer to the demands of the dominant, which has already been explained here. So M is correct in that descending without the flat 6 and 7 makes it an improper melodic minor. But … composers need to get used to thinking about tetrachords, rather than pure scales, to get the necessary harmonic facility when more than one voice is used. Think of tetra chords as the first and second half of scales.