if an expense is paid for service contract for one year
how the account to be journalised

When you pay the amount of the contract you should record the following entry:

Dr. Prepaid expenses
Cr. Cash

Then at the end of each month you should amortize the portion of prepaid expense to an expense account by recording the following entry:

Dr. Services expense
Cr. Prepaid expense

Lets assume that you paid an amount of 12,000 usd on the first of december 2008. the entry at that date will be as follows:
Dr. Prepaid service expense 12,000
Cr. Cash 12,000

At the end of December 2008 you should record the following entry:
Dr. Service expenses 1,000 (12,000/12 months)
Cr. Prepaid service expense 1,000