Welcome to FT!

I hope others here, many of whom know a lot more than me, will correct me as needed, but here is my advice.

Generally, US-based carriers have better FF program, even if they don't operate the most pleasant flights. Therefore, whichever airline(s) you'd be flying, find out which alliance(s) they belong to and sign up for the FF program(s) of a big US-based carrier in that alliance. So, Singapore belongs to Star Alliance and you might join United; when you fly Singapore, give them your United FF number and have the miles credited to United.

Similarly, if you fly Cathay Pacific, they belong to One World Alliance, and you'd join and have the miles go to American. Etc.

Second, it is good to bank your miles at one airline, and only fly its alliance partners. Otherwise by spreading your miles around you may never earn enough to do much with them. Do some research on which airline(s) tend to have good schedules and prices for your routes and choose accordingly.

Third, the best way to prevent miles from expiring is to have some routine activity going on in the account. One good way is to get a credit card affiliated with that airline (which is home to your miles, United in the above example). Also, familiarize yourself with your airline's shopping partners, eg, banks, phone companies, online retailers, car rentals, even restaurants. This way you'd be earning a few miles every now and then which ought to be enough to prevent your miles from expiring.

BTW, I am with AA/One World; that's all I know about and everything else is just a common sense extrapolation. However, if a Star Alliance person gives you different advice about their world, please listen to them, not me.

Good luck.