Each brand may differ slightly but here's a guide to finding the right shoe size for you. Clothing sizes can be found here.

Mens Shoes:
UK 5: USA 6: European 39
UK 6: USA 7: European 40
UK 7: USA 8: European 41
UK 8: USA 9: European 42
UK 9: USA 10: European 43
UK 10: USA 11: European 44
UK 11: USA 12: European 45

Converse use their own size chart where both UK and US sizes are the same.

Womens Shoes:

UK 3: USA 5: European 36
UK 4: USA 6: European 37
UK 5: USA 7: European 38
UK 6: USA 8: European 39
UK 7: USA 9: European 40
UK 8: USA 10: European 41 

Converse use their own size chart where both UK and US sizes are the same.

Kids Shoes:

UK 4: European 21
UK 5: European 22
UK 6: European 23
UK 7: European 24
UK 8: European 26
UK 9: European 27
UK 10: European 28
UK 11: European 30
UK 12: European 31
UK 13: European 32
UK 1 (youths): European 33
UK 2 (youths): European 35 

Kids Age Guide:

Infants

3-6 Months = Size 2
6-9 Months = Size 3/4 
9-12 Months = Size 4/5 
1-2 Years = Size 6/7/8
2-3 Years = Size 9/10

Juniors

4-5 Years = Size 10/11
5-6 Years = Size 11/12
6-7 Years = Size 1/12/13
8-9 Years = Size 4/5

Clothing Sizes

Mens Clothing:

Small (2): Chest 34-36. Waist 28-30
Medium (3): Chest 38-42. Waist 32-34
Large (4): Chest 42-44. Waist 34-36
XL (5): Chest 46-48. Waist 36-38
XXL (6): Chest 50-52. Waist 40-42

Womens Clothing:

Size 8: XS. Bust 30-32. Waist 23. Hips 33. USA 6. Europe 38.
Size 10: Small. Bust 32-34. Waist 25. Hips 34-36. USA 8. Europe 40.
Size 12: Medium. Bust 34-36. Waist 27. Hips 36-39. USA 10. Europe 42.
Size 14: Large. Bust 37. Waist 29. Hips 40. USA 14. Europe 44.
Size 16: XL. Bust 39. Waist 31. Hips 41. USA 16. Europe 46.
Size 18: XXL. Bust 41. Waist 33. Hips 43. USA 18. Europe 48.