More detailed information about money is included in the Budgeting section.
You should arrive with some small amount of currency for that country for lay-overs and missed connections. Most airports and train stations have ATMs and you can get cash through these. The currency exchange kiosks tend to have a high rate for exchanges.
Traveler’s checks are not easily cashed in some areas because of the high fraud rate.
Consider using RFID-proof wallets or card slips to deter thieves from scanning your cards while you carry them in your wallet.