Do you mean you want to complete your undergrad here, then do postgrad overseas, e.g. in Singapore? Or, do you mean you wish to transfer to a university overseas having only partially completed your undergrad degree here?

The first option is quite simple to achieve. It would be best if you could enter one of the best public universities (i.e. the University of Malaya) as it would expedite and ease your admission into a good school overseas for your graduate studies. To do so, I think you need to do STPM.

For universities overseas, you can do either STPM or A Levels for undergraduate entry. Its excepted in most major institutions in English-speaking countries and most English-medium universities.

For the second option, this is difficult to achieve as it is less likely you can transfer (as a continuing undergraduate student) from a much lesser school to a much better one, unless you are studying a specific degree transfer-programme or external programme which allows transfer.

Normally, before all of that comes STPM/A Levels/IB.

However, certain courses (like ADP/ADTP) can facilitate transfer using pre-university credits in specific cases.

Well, taking A Levels/IB/STPM means you will (with good results) be eligible for consideration for entry to many universities overseas, and be accepted to some of the best of them. Of course, other things you could do are, like I mentioned, ADP, or other Pre-Us (CPU, SAM etc, not recommended), or Foundations (not recommended), or Diplomas (not recommended).

It is much easier to gain admission into a good university in Singapore or Britain, Ireland, Australia, Hong Kong, Canada etc (I mean a good one, at least top 300 in the world or better, maybe in the top 100 in some rankings) using a recognised Pre-U such as A Levels/IB/STPM as compared to a less accepted (or less favourable) Pre-U/Diploma. Foundations are often specific and highly limiting. Since you have high aspirations, avoid these restrictive courses. Transfer Programmes are more flexible than Foundations, but they are only good if you know where you could and/or want to go (i.e. the specific school or group of schools).

Like some people said (but this I know nothing about), you need STPM to enter local public universities - so take A Levels if you believe you'll head overseas for your degree.

You should take Further Mathematics. Mind you, the only undergraduate degrees I have ever seen that require A Level Further Mathematics or equivalent are Mathematics and joint/mixed degrees with Mathematics (i.e. Mathematics and Statistics or Mathematics with Statistics, Mathematics and Statistics for Finance, Mathematics with Computing etc, but not Statistics or Computing on their own). Although, I have only ever seen this requirement at less than 4 schools.