For any sort of doctor’s visit, therapy session, or even insurance payments an invoice (receipt) with proof of payment (debit/credit receipt, or payment applied on statement) will be more than enough.
In order to claim your prescriptions you will need the RX numbers which is usually attached to the prescription so be sure to keep those. Many pharmacies can print you off a yearly summary of your prescriptions and payments so feel free to ask them if this is available (it’s easier to keep track this way).
Insurance payouts can be difficult to keep track of, be sure to keep all information in regards to payments, or payouts. Many insurance companies can print you off a summary of your yearly activity outlining any medical claims and payments.
When it comes to your medical expenses the more information you keep, the better off you’ll be.