Once your wallet has been created, obtain your receival address starting with "addr..".
Go to your nominated Centralized Exchange, purchase $ADA and withdraw to the new receival address acquired - make sure you do not modify this address, and double-check before submitted your withdrawal request — if any errors are made, you are at risk of losing funds.
Now that you've installed a new wallet, and added funds — load up one of the following websites and press on 'connect wallet'.
Once connected, you can now commence trading using their swap features.