Hi, and welcome.
Do you mean that you had it working previously and it stopped? If you do, try going into settings, and clearing the Playstore app's cache and data, then reboot the device and try again. That usually fixes it.
if you have a device that does not have Playstore on it, it is a bit more complicated, but not impossible. Your best bet would be to find your device's folder on forums and ask there, as each device is different. If it did not come with Playstore, someone *has* come up with a way to put it there.
Hope this helps.