Etherscan – Etherscan is useful for viewing accounts and transactions on the block chain, among other things.
Remix – Remix can be used to compile and deploy contracts. It can also be used to interact with an already-deployed contract.
MyEtherWallet – MEW is another tool that can be used to interact with deployed contracts. You'll need the ABI for the contract, which you can get by compiling it.
Things to read
Program the Blockchain – This is where I blog about smart contracts, so as you can imagine, there's a lot of useful content there.
Solidity documentation – The Solidity documentation isn't always the best written, but it's fairly comprehensive.
The yellow paper – Notoriously hard to read, the yellow paper is nonetheless the definitive source for details about Ethereum and the Ethereum Virtual Machine (EVM).
Code libraries
web3.js – web3.js is the standard JavaScript client library for working with Ethereum.
ethereumjs – The ethereumjs suite of Node.js modules are quite useful.
web3.py – This is essentialy web3.js but for Python.
Ways to get help
/r/CaptureTheEther – I've set up a subreddit where people can ask for help, but please avoid spoilers!
/u/smarx – Feel free to reach out to me directly on Reddit via private message or chat. I'll do my best to get back to you quickly.