Plastic food containers are ideal to use as moulds because they are flexible, and it's easier to remove the soap from them.
Metal containers are rigid and removing the soap from them will be difficult.
Never use aluminium moulds, because the lye will react with the aluminium.
Wax coated paper milk cartons, cutlery trays, plastic takeaway food containers all make good soap moulds for a beginner soapmaker. You can also use large plastic food containers for the entire soap mixture, then cut the soap into blocks later.