Where to find past papers:

Page on xtremepapers.com

If you want past papers for Edexcel IGCSE this site is better
ALL IGCSE, GCSE and A LEVEL PAST PAPERS AND MARKSCHEMES (BETA) It has the most recent ones on there.

This is useful for biology questions – more relevant to CIE.
Biology GCSE and IGCSE Question Bank, questions for self-assessment or teaching, by D G Mackean

For CIE IGCSE past papers

IGCSE centre

 

Where to find simple explanations to help you understand the content:

This is a good site for Chemistry, but I find it too stressful to look at – it’s very ‘busy’!
GCSE CHEMISTRY REVISION NOTES IGCSE O Level student exam revising study help doc brown iphone ipad notepad notebook netbook epad tablet android

Also this site for Edexcel chemistry revision resources
chemistryatdulwich – IGCSE Chemistry

Whatever you look at make sure it’s relevant to your specification – there is a fair bit of overlap between CIE and Edexcel but you don’t want to waste your time learning to wrong explanations/ irrelevant content.