When you have started the match and want to report your first goal, go to the Events tab and tap the green REPORT EVENT button at the top of the timeline. 

You will now see a list of different events to choose from. Tap on Goal.

You now get to specify which team who scored and what kind of goal it was.

The next step is to fill out the goalscorer and the assist-maker.

Once this is done, press the green SAVE button at the bottom of the page.

Now your goal should appear on the timeline under the Events tab.