How to test Uniswap on testnet
You can find uniswap on all testnets. Let's try switching metamask to KOVAN network
To get some ETH go to https://faucet.kovan.network/ or https://enjin.io/software/kovan-faucet or just try googling for KOVAN FAUCET yourself
to get some DAI put CDP collaterized dept position on https://oasis.app/borrow?network=kovan
Now if you have ETH and DAI (or any other two cryto tokens) go to uniswap, connect with KOVAN network and start playing and learning with free money. Exchanging tokens or provide liquidity on mainnet is quite expensive with current gass fees
When you will start staking on mainet have in mind the IMPERMANENT LOSS
Tool for calculating impemanent loss: https://dailydefi.org/tools/impermanent-loss-calculator/