Finding sums due to HMRC including payments on account for those in MTD
Finding sums due
The amount you owe HMRC can be found in the Making Tax Digital section, under Sums Due.

In the mobile app this can be accessed via the Taxes tab.
Note that if you are in MTD and are looking in HMRC's own systems, then the sums due may appear in two places - under self assessment and in the MTD section.
Paying HMRC
You can pay HMRC via this link in the untied in the browser:

You can also pay via HMRC systems.