suppose im playing pretty basic chords like Em-C-Am-D.
what modes can i improvise with ? how do i decide ? do i need to alter the chords in order to play in a specific mode?
You're basically locked into Aeolian for that progression - so E minor ( or Aeolian). You can also play E minor Pentatonic, and even maybe squeeze in some E harmonic minor. That being said - that progression is the quintessential straight up minor progression.

do i need to alter the chords in order to play in a specific mode?

Yes, you would need a different chord progression to play a different mode. For example, you could play E minor, D major and A major - then play E Dorian over that whole progression.

Also, if you play Em, C, Am, Dminor - you could play E phrygian over that progression. The progression dictates what works and what doesn't.
That progression - or any technically proper progression - isn't really conducive to usage of modal scales. You'd do best to find a backing track where the chord changes are slower and fewer. If you go to Youtube and just search for something like "dorian backing track" (or phygian or whatever), you'll get plenty of results.
A really simple way of creating a modal sound is to play the IV and V chords of major, using a bass pedal with ii, iii, IV, V, vi or vii in the bass.

e.g. starting with C major, IV and V are F and G major. So try F/D, G/D (D Dorian sound). F/E, G/E (E Phrygian sound). F/F, G/F (F Lydian) and so on.