Do not waste your time buying an A-level text, or revising from one, you will just cover lots of stuff you do not need, and waste half of your time.
Well, the content of mathematics in the first year varies depending on the uni, so if possible, look into the module catalogue.
But as general advice, get
K.A. Stroud Engineering Mathematics, and work through that (I have worked through about 1/3 of it in my holidays so far...), it is the most popular book for 1st year mathematics, and covers most topics for first year courses (except fourier series, etc...).
Subjects...well as I said depends on the uni but the basics generally are:
- Trigonometry (identities, equations, double angles, compound angles etc)
- Hyperbolic functions & equations
- Even, odd, inverse functions of various things.
- Integration (areas under curves, volumes of solids of revolution, mean, RMS, by parts, by algebraic substitution, by partial fractions, etc.)
- Differentiation (up to optimisation, implicit, parametric)
- Complex numbers (from the basics through to de moivres theorem, exponential form)
- Sequences and series (binomial expansions, APs, GPs)
- Determinants to solve simultaneous equations.
- Vectors in 3 dimensional space.
- Limits by inspection.
- Newton Raphson iterative method.
If you know that stuff, then do:
- Fourier series
- L'Hopitals rules
- Maclaurin and Taylor series
- Differential equations
- Multiple integration, other harder integration stuff.
- Differentiation of multi-variable functions
Hope that is of help to you.