Most countries that require visas, can be applied online, just
do a simple search and check if as a singaporean, you need a visa
to visit that particular country. You need a credit card to make
payment.
Travel agencies can help you apply for it as well, but you need
to provide them alot of personal details, not just your name and
passport number, and sometimes they do charge for it, as its extra
service. They will normally indicate they will not be responsible
if your visa fails to go thru, because there is no refund for a
failed application, and then you will have plenty of issues with
the agency, which ain't nice.