How to generate realistic echo to sounds?
A very simple model is a sum of attenuated and delayed replicas of the signal. Such signal can be be generated using FIR filter. The length of the filter is the "echo time", i.e. how long the echoes last.
The simplest approach would be to generate sparse impulse response. Build a filter with $n$ non-zero coefficient and fading amplitude (for example the amplitude can be generated with Rayleigh distribution).  

If you want a more realistic scenario, I suggest using the Room Impulse Response (RIR) generator by Emanuel Habets.