After installation, Brave gives you a small amount of unwithdrawable BAT mainly for you to test out by giving to publishers/websites(and not for withdrawal). If your country is already supported, you can then activate Brave Rewards via the settings, and can earn withdrawable BAT from there by simply viewing ads that are displayed when you're randomly surfing the web.

Note that withdrawing the BAT you've earned through Brave Rewards requires you to send KYC info to Uphold. Just a heads up.