How can I write better English?

A good starting point is Strunk's classic text on the "Elements of Style". The first edition is available online:

Strunk, William. Elements of Style. Ithaca, N.Y.: Priv. print. [Geneva, N.Y.: Press of W.P. Humphrey], 1918;  1999. www.bartleby.com/141/.

For a very clear, easy-to-use and comprehensive  survey of all the common grammatical errors and how to avoid them, we recommend:

Swan, Michael. Practical English Usage (Oxford University Press)