You MUST have your BTC in your own private wallet, so start by downloading Bitcoin Core, which is ideal.
If you don't want to download the whole blockchain, get a paper wallet, or Electrum wallet. Anything where you control your private keys, otherwise you will NOT receive the coins on the other chain.
So if we do not download the whole blockchain, or get a paper/or Electrum wallet, (to control private keys) does that mean we lose everything? lose the opportunity for the fork? what is worst case scenario. Seems very confusing.