Samsun is a city in Turkey.
Take the tram to the endstation called 'Gaz'. Walk further until you see a big pedestrian-only bridge on your left side (it says Buyuksehir sizin icin). Walk even further until you see the big hotel and shopping complex called 'Anemon' to your right. Walk even further until you cross a sport-complex (with football and tennis) and a park on your left side, called Dogu Park. Note that all the time of this walking, you will walk parallel with a motorway. After maybe 2,5 km walking, you will see the pillars of a motorway that is not yet finished. Turn right, and walk under these yet-to-be-finished motorway. This is where all the traffic goes through that will surpass Samsun. Start thumbing after the construction sites.
Yes, it is a long way from the center but once you're at this very spot, direct (truck-) rides to Ankara and Istanbul and waiting times less than one minute are not uncommon!
East towards Trabzon
Take a bus going to a replica of Bandirma ship (Ataturk came with this ship to Samsun which marks a beginning of the Turkish War for Independence). Everyone knows it. Bus 17 or 19 go there (ask the driver to be sure). The ship is directly at the Eastern roundabout of the city where you can thumb.
See the part above about getting out of Samsun in direction West. Dogu Park is actually a very nice place to pitch a tent, with locals barbeque-ing there on hot summer evenings and children playing. Some locals might have brought there own tent and have set it up in the park too. Police is completely fine with people sleeping in tents in this park. It is just a very cozy place.