The most important things you need to do in Maths are to add, subtract, divide and multiply. If you’re doing an entrance exam, and there’s more than one mark for a question, it generally means that you have to show your working. Even if it’s easy enough to do in your head, you still have to write down the sum on paper. That way, the examiner knows that you didn’t just guess!
Visit Nick Dale's website for a step by step guide with images to help you follow along
BBC's guide to breaking down numbers and adding in your head - great for mental arithmetic practise!
I like Khan Academy and they are a no-nonsense learning tool made simple, with videos to help!
Explore adding and subtracting with numbers below zero with Maths Made Fun! A basic and easy to understand revision site.