If your CPU has the PCA extension, its even easier than using the normall timers. The intel PCA manual mentioned in the links page is full of handy tips.

Otherwise, what you want is achieved within the "ISR", called as the timer overflows, and generates an interrupt (if you enabled it in the appropriate control register, and you enabled EA). The will need you to calculate a timer reload value for your particular clock frequency, so that it can start counting at the right interval next time round.

If that's all going over your head, you really need to read the (in)famous "8051 bible" - section 1 is the most pertinent. - also in links.

Steve