In your Moodle course, click on "Turn editing on" in the top right corner of the course page.
Under the category in your course where you'd like the assignment to reside, click "Add an activity or resource"
Click the check circle next to "Assignment", then click "Add".
On the next page, you'll name your assignment and apply other settings.
Under the "Grade" section, you can configure a Maximum grade. This is how many points the assignment will be worth in your gradebook.
You can also change the Grading method. The grading method you choose will depend on the assignment, however most essays or responses will work best with "Simple direct grading". Using the "Rubric" may be better suited for assignments with multiple parts.
You can also set due dates, choose how you want students to submit assignments, and change how feedback is given to work.
Once you've configured your settings, scroll to the bottom of the page and click "Save and display".