How to choose the best background for your painting?

Its depend about your theme.

which type of painting you want to create ..according to that only we can help you

What is behind the subject? Paint that.

Thank you for your advice, the creative process still need to continuously grope.

I suggested as I did because it sounds (and I could be way base here) that you are a beginning art student or hobbyist(?). Making your background dull and lifeless (as with a blanket or blank wall or other) will enable the viewer to appreciate the foreground and subject that you are attempting to capture. As your talent matures, so will your backgrounds. Adding in landscapes or fading colors or Pollack like splatters will fill the background but keep in mind what you want your audience to see and react to.

Hi there, best background can be chosen by your choice of Interest. All you got to do is just think about the topic and story of your painting and chose the background accordingly. Simple!!

I'm no pro, but, whenever I've dabbled in paintings or even making posters, I find that using the colourwheel is really helpful.

You draw and colour your subject first, then you look at the predominant colour you've used on that subject - if it's a human, it might well be that the predominant colour is the one you used to do their clothing as that tends to cover the biggest area of a human subject. Whatever the subject, assess what the predominant colour you've used is and then use the colourwheel to find complementary colours for the background to really make your subject leap off the page (complementary colours are the ones opposite your subject's predominant colour on the wheel below).

Then, you just have to choose a background that would naturally have those colours.

For example, say I painted a madonna and she's wearing the traditional blue garb. She's just floating there now on a blank white canvas background I haven't painted yet. I need a nice background. I look at the wheel. I see that blue is complemented by oranges, reds and yellows. What background might use those colours? I know! I'm going to go for a landscape with a lot of sky and that sky is going to be at sunset (red, yellow and orange).
That should look good against my blue subject and really make it jump off the page.

BUT

Alternatively, I might want my painting to be more sedate and less aggressive on the senses. I might not want the subject to leap off the page. If so, I choose the colours immediately next to your subject's predominant colour. So, if I want my painting of the blue Madonna to feel more relaxing and calming, I see that greens, blue-greens and violets are closely related to my blue. What background might use those colours? Could go for a field at night-time? Simply a window with a violet velvet curtain and a green field beyond? The sea? Anything that uses those related colours really. I know those colours will produce a soft painting.

So, to summarise -
Calm painting = background using colours next to my dominant subject colour.
Vivid painting = background using colours opposite my dominant subject colour.

Again, I'm no pro, but I usually find this works for my tastes.

Hope this helps! Post your results, please. Would love to see your finished work.

Thinking twice, and doing it carefully. Try to imagine what you want to paint.