How to get app to look the same on different devices

Hi there, If anyone has some advice on this it would be much appreciated. I have completed my app and am ready to launch on google play. However I made it while using an iphone to check on my progress.

Once I was able to get a hold of an android I noticed none of the buttons where the size they are meant to be and it just over all looks bad.

How do I guarantee that a page will look the same on every device? There is no scrolling involved every new page should just take up a page.