It's easy, you can even start a sample API query from your browser window. Just paste use this sample query to get the RSI value of BTC/USDT on the 1h timeframe:
https://api.taapi.io/rsi?secret=APIKEY&exchange=binance&symbol=BTC%2FUSDT&interval=1h
Just don't forget to paste in your API key where it says APIKEY.