how to start a garden that attract butterflies?

Asclepias tuberosa. It's a beautiful orange-flowered perennial that is hardy to at least zone 4b. I've grown in successfully for years. It's a relative of the common Milkweed that you see growing on the sides of the road, although it's much prettier.

http://en.wikipedia.org/wiki/Asclepias_tuberosa

Parsley & Dill. One year, I had a bunch of hungry Black Swallowtail caterpillars devouring my crop of parsley and dill, but I didn't mind the sacrifice. They later pupated on my Echinacea plants.

Lantana camara. It's a tropical tree that you've got to bring indoors if you live in a cold winter climate, but it flowers consistently all summer long and Monarchs especially love it.

http://en.wikipedia.org/wiki/Lantana

Anything in the Asteraceae family:

Purple coneflower (Echinacea purpurea),

http://en.wikipedia.org/wiki/Echinacea_purpurea

Yarrow (Achillea millefolium),

http://en.wikipedia.org/wiki/Achillea_millefolium

Globe Thistle (Echinops ritro),

http://en.wikipedia.org/wiki/Echinops

Black-eyed Susans (Rudbeckia fulgida).

http://en.wikipedia.org/wiki/Rudbeckia_fulgida

I live in zone 4b and I grow all of the above successfully without too much effort, except for the Lantana, which as I mentioned, I have to bring indoors during the winter months.

Unfortunately, I don't know what flowers would be best for your region, but a couple things to remember:

If you want to attract butterflies, you need to plant both nectar plants for the adults and host plants for the caterpillars. Sometimes these are the same plant, like the milkweed here in America, which is a host plant for Monarch butterflies and a nectar plant for monarchs and many other species, but often they are not.

It's important to plant mainly native plants in a butterfly garden. Caterpillars can eat only from one family (sometimes even one species) of plant, and these host plants are almost always native. Butterflies are not as picky about whether a plant is native or non-native as caterpillars, but many show preferences for native plants.

Aim for diversity in your plant selection. The more kinds of host plants you have, the more different kinds of butterflies. Choosing flowers so that you will have at least one species blooming at every point during the growing season is the best way to get butterflies to stick around.

Try to avoid double flowers because often they don't contain much nectar. Also try to avoid F1 hybrids and other plants where there has been an intensive breeding program. In Singapore members of the ginger family are a good bet but what you really need to do is visit the Singapore Botanic Gardens (64 hectares of enormous interest between Cluny Road in the east and Cluny Park Road in the west) and make careful observations. See what the insects are going for.

Butterfly timber are the excellent. there is likewise a perennial flower with reddish orange vegetation stated as a butterfly flower. that attracts them extremely nicely. attempt choosing vegetation that have a collection of little vegetation clustered mutually in bunches. Butterflies tend to love those forms of vegetation. in addition they look to love Echinacea and Gaillardia vegetation even if no longer besides as butterfly timber. I constantly have a ton of butterflies each and each 365 days with butterfly timber. i spotted on the butterfly gardens they placed out orange slices for them to devour on.

Echinacea Wiki

In UK the common Budlia shrub is also known as the 'Butterfly bush' as it attracts them for sure. I imagine it will grow in Singapore. It grows everywhere here, even on derelict land.