So i learnt that u first hit the chord, and then hit the sustain pedal

you take off the sustain pedal as soon as you hit a new chord and press is on again



but i was wondering how do you use sustain pedal if you're playing a melody?....

do pianists use sustain during fast melodies for example or is it that not needed?

 

I have no conscious explanation for how, why or when I employ the sustain pedal, when I slightly bend a string or add just a touch of vibrato or modulation to a note or chord for that matter. I never ever thought about any of this before now. It has just been something that has developed over time these last 50 years of playing musical instruments. I learned various techniques and practiced until they all became part of my musical personality and voice.

Don't overthink it, just keep on playing. It will all make sense sooner or later and you are playing in the moment.

 

FOR PIANO it depends on the piece you're performing, but it's not perfect science.
Usually you can find on scores the Ped * mark, but even if there is not, legato mark gives an idea how a phrase should be pedalled.
Here a pair of examples (Chopin - Fantaisie Impromptu and Debussy - Clair de Lune).

For synths, it really depends on sound, (I wouldn't abuse pedal on a continuous sawtooth lead, it's a mess), polyphonic capability (note stealing) and your feeling. Practice over and over and you'll find the limits.