What's the best way to take a picture when it's dark?  

The best way would be to use a flash. You may have bought it along with your camera or may buy it separately. There is even a better way and that is to use a floodlight kinda thing that all those professional photographers use to shoot in dark.

You're right, you can adjust the ISO but I usually use that as a last resort since anything above 400 makes the picture grainy.

It would be a good idea to practice Manual since I believe if you have that down pat, you're good to go.

For Aperture priority, you might want to use a bigger aperture (the lower the f-number, the bigger the aperture) to let more light in. If you don't want to use flash, better have a good tripod handy and use a slow shutter speed to let more light in. Since you're using slow shutter, your picture will be brighter but more prone to motion blur so your tripod should help.

Once you get the hang of Manual, you can adjust both shutter speed and aperture to arrive at a well-lit picture. Don't forget to use a tripod or at least keep your camera steady!

Dark images are usually caused because at the time of photographing the light intensity is less so the photos that we take becomes dark. solution it, which I do is edit image it through fhotoshop

I think it depends on what kind of shot you want. I usually don't use a flash as I don't have an external one, just the inbuilt one on my camera and it is not really very good. So if there's something I want a picture of and grain isn't an issue, I increase the ISO til whatever value I need. My camera takes quite OK pictures at 800, higher is OK as long as I don't make bigger prints than 10*15 cm. If I make bigger, I'll usually turn them into black and white so that the grain looks more natural or artistic ;-)

If you want to take pictures in the dark of things that are standing still your best bet would be a tripod or some other way of holding your camera very still.

Or you can get lenses with image stabilizers, then you can get at least two extra stops before the shots starts getting blurry.

Welcome to mylot
When dark i always go for the auto flash ootion with my canon 200 and Fujifil digital cameras, if too dark forced flash may be requied..
Thanks for response, God bless you.
Cheers
Happy posting.
Prof