First of all you're going to need to head over to the settings tab and click on the forms button.
This will bring up a list of any forms that your project has (if any). click add form.
That will bring up a slide out menu where you can name your form.
In the Data Definition drop down you can choose what type of form this will be. (There may be slightly different lables on your project.)
submission will be will visible to submitters.
Standard Event Review Data Definition will be visible to reviewers.
After you've chosen your data definition, click add form and your new form will be added to the bottom of the the list.
Your form has now been created but it will be blank so we are going to have to add to it.
Click Edit to start adding to it.
this will bring up the the form editor.
On the side panel you will find four tabs with everything you need to set up your form.
Form Details: here you can change the name of your form, add a page and change the settings.
Edit Page: This tab is where you can label the page. you can also see how many fields are on the page.
Edit Field: here you can edit the fields you have added
Add Field: Here you can add fields to your form by clicking on the items listed.
For more infomation check out the guide to editing a form.