I cut my foot open on a lid this afternoon. what's the best first aid?

Go to ER or PCP: You need to seek medical advice-er, pcp, etc. The most important thing is when was your last tetanus shot and are you up to date. Second, is preventing bacterial infection. The laceration needs to be thoroughly cleaned with saline and depending on size/depth of lac. Possibly sutured. Best to have it evaluated by a doctor or nurse.

Several things: Wash the foot with warm soapy water and apply an antiseptic and bandage to keep the cut edges together. If the lid is dirty you need to make sure your tetanus shots are up to date. If deep it needs to be examined and possible stitches put in.