Start from zero — the basic math you need before everything else
Every number you write is built from just ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. A digit's size comes from where it sits. The same digit 5 can mean five, or fifty, or five hundred. It depends on its place.
The places line up in columns. Starting from the right: ones, then tens, then hundreds, then thousands. Each column is worth ten times the one to its right. So in 372 the 3 is in the hundreds column (300), the 7 is in the tens column (70), and the 2 is in the ones column (2). Add them up: 300 + 70 + 2 = 372.
Think of money. To make £372 you'd grab three £100 notes, seven £10 notes, and two £1 coins. The same note is worth a different amount depending on which pile it lands in — and that pile is its place. Hundreds, tens, ones: exactly the columns above.