Using a Graphical Calculator
The rigours of A-Level maths mean that you will need a more advanced calculator that will perform functions such as solving linear simultaneous equations, solving quadratics, performing calculations on a binomial or normal distribution and many others.
Within thge casio range of products, there are two options available for you, either a classwiz fx-9911ex or a graphical calculator which would be one of the cg50 or the cg20.
If you can afford the extra cost, I would recommend investing in one of the graphical ones. The graph mode has some incredible features that can be gold-dust on many topics.
Whichever you buy, you must practice using them. Whenever you complete an A-Level question, ask yourself 'How can I check my answer using my calculator'.
Practice will make perfect!