"What are the job opportunities for students of economics? I have never studied economics in the senior high school but can I major in economics at university because am good in maths?"
Great question.
Nearly two-thirds (63.9%) of economic graduates from 2002 were in employment six months after graduation, so the prospects are great. Some economics graduates get employed as economists in either the private sector or the public sector. Companies and government departments require economists to be able to forecast how the economy will impact on them and to help them make the right spending or purchasing decisions. Employers also like to employ economists for the analytical skills that they bring rather than employing them as practising economists.
You are also correct in thinking that being good at maths helps with economics. Modern economics does make substantial use of mathematics. All economics degrees will require you to be comfortable in handing data and in learning various formulae. On joint honours degrees the level of maths is relatively low and goes little beyond basic algebra and arithmetic. Therefore some, but not all, economics courses require A-level Maths (or equivalent) and virtually all require GCSE Maths at grade C or B. For individual details it is best to check with each particular university. As far as not studying economics at senior school, to our knowledge, not one university requires economics at a-level. Hope it helps.

