How to use A4 instead of letterpaper?

The article class has an option for A4 (210 mm x 297 mm) paper:

\documentclass[a4paper]{article}

other possible options for page size are:

• a5paper - A5: 148 mm x 210 mm, that is half of A4, basically
• b5paper - B5: 250 mm x 176 mm
• letterpaper - Letter: 215.9 mm x 279.4 mm
• legalpaper - Legal: 215.9 mm x 355.6 mm

You should also have a look to KOMA script, instead of the default article class:

\documentclass[a4paper,10pt]{scrartcl}