Świecko border crossing
It is also the name of a village close by.
Directly at the border there is a suitable place where cars drive relatively slowly and where it's quite easy to get a ride. There are some shops, currency exchange offices and Polish Customs. In summer there might be quite a lot of dust in this place - be careful if you have an allergy.
Supposing you didn't hitchhike to Świecko, you can get there easily from the Słubice railway station. When you get out of the station, go left (southward) instead of right (as to the city of Słubice). You will soon see a lot of trucks, simply follow them and you will get to the checkpoint at the Polish-German border.
You can find a driver to Świecko quite easily along the A2 for example in Michendorf. From the border crossing you can easily catch rides to the bigger cities in Poland, but with some luck you could even catch a ride to Lithuania, Ukraine, Belarus. Sign advisable.
If your driver is going to Berlin and you want to hitchhike further here is good place to get dropped. Sometimes, most cars seem to go only to Berlin (in the evening/at nightime?). It might be even better to get out at the last petrol station in Poland a few kilometers before, because most drivers stop there to refill cheap fuel and buy cigarettes. Don't accept a lift to the next ("Biegener Hellen") service station in Germany (15-20km), very few people stop there.