I was denied entry to Poland last February. The border guards told me this was because I didn't provide enough supporting documents like hotel reservations and travel plans. Will this affect my chances of entering other European countries in the future?
Yes, this will affect your chance to enter any other European countries, and probably some non-European countries as well. But it won't normally result in an automatic refusal.
A past refusal of entry will certainly serve as a red flag to any visa officer or border officer who is evaluating whether to grant you a visa or allow you into a country. But if the reason for a refusal is simply a lack of sufficient supporting documents, it shouldn't prejudice a future decision maker against you.