In addition to the columns you already have (client name, address, date
etc), you might need a column for the annual amount they are committed
for if this varies for each client. In another column you could record
whether they will pay annually or monthly ("A" or "M" - maybe also "Q"
for quarterly).

Then basically you will need 12 columns, one for each month and a
formula copied across and down which will take account of the
start-date, the payment frequency and the annual amount to determine
the income for that month - presumably annual and quarterly payments
are made in advance?

Finally, you could total each column to determine the projected income
for each month. If you want to add more clients as the year unfolds,
you might like to put the totals at the top of the sheet and use a sum
range beyond the number of clients you currently have.

Hope this helps.

Pete