Men's jeans use waist x inseam (e.g., 32x32). The first number is your natural waist in inches, the second is your inseam.
Women's jeans use either numeric (00, 0, 2, 4…) or waist inches. Numeric sizes correspond loosely to waist: 0 ≈ 24", 4 ≈ 27", 8 ≈ 29", 12 ≈ 31".
Measure your natural waist (above belly button), not where your current jeans sit.
For inseam, measure a well-fitting pair of pants from crotch seam to hem.
Different cuts fit differently at the same labeled size. Skinny fits tightest, relaxed fits loosest. Always check the leg measurement.