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.
There is a small potugese fort close to Mandrem, you will see it as soon as you walk out on the beach, getting there is quite easy, free freey service is operatet every 30 minutes but the walk is quite exhausing especially if its hot, but will give you the chance to see a sleepy small vilage were tourists still are quite an atraction. The is an abdomned school-building in the woods just shortly bevor entering the village, an almost impassable small path lead there and these 5 minutes walk are really worth it. The Fort is now turned into a High-Class hotel but the stuff dont mind if you walk around and enjoy a beautiful view over mandrem beach and Maharashtra in the north.
Exept for this, there is little to do than laze around at the beach or take a swim in the suprisingly deep water.
Visit the beautiful deserted beach, just a few huts offering not-so-good food, but the beach and sea are worth it.