Insurance is optional, think of it as "If your car isn't disposable then get some". Helmet is a requirement although most trackday companies will let you hire one for £10-15 or so which for a one off will be better value than buying one. Also some circuits are starting to insist on "motorsport approved" lids which tend to be more expensive then the "cheap bike lid" option that many start out with. Best to check with whoever is organised the day you are thinking of attending.
Mechanically the main thing is to ensure that your brakes are in good order. You won't necessarily need to uprate your pads (depends on the circuit, your driving style and how long you spend out there without coming in to let things cool off) however you probably will take a fair bit of life out of them so depending on how old they are you may find yourself in need of some new ones by the end of the day.
Tyre wear will depend on the circuit, as a generalisation an airfield day (which tends to be on rougher tarmac and concrete) will be harder on tyres than a proper smooth tarmac race circuit. Some very soft high perfomance tyres can really struggle with overheating on a trackday (Eagle F1s are notorious for this) and you can easily wreck a set very quickly. If you're looking to do a day in the next few months then you may get cooler and/or damp weather which will help here.