How do I build my first trading algorithm?

Start with one rule you can say out loud, like 'buy the S&P 500 when its 14-day RSI drops below 30, sell when it recovers above 70.' Then: (1) express it in a no-code builder or plain English; (2) backtest it over several years of data and read every trade — not just the headline return; (3) compare it against simply buying and holding the same asset over the same window; (4) paper trade it on live data for a few weeks; (5) only then connect real money, small. The biggest beginner mistake is skipping steps 2–4 and going straight from idea to cash. The second biggest is tuning parameters until the backtest looks perfect — that's overfitting, and it evaporates live.