Mandrem is a city in India.
Busses go from Pernem a few times every day via Arambol, its the last stop of the Line so don't worrie about missing your station.
If you looking for an adventure, you can walk along the beach-side from Arambol, even if the walk is more like climbing due to the steep coastline and the difficult terrain, so don't even think about doing it with you backpack on.
A mototaxi from Arambol will cost you 100-200 Rupees.
From the small vilage of Mandrem just walk towards the sea, there is little chance that you will miss it.
Visit the beautiful deserted beach, just a few huts offering not-so-good food, but the beach and sea are worth it.