There are a few ways to do this.
1. If you have a lot of content items you created, or brought in from a course copy, you can make them all visible at once via the batch edit option. Click the ellipsis in the upper right hand corner of the content area and then click Batch Edit:
On the next screen just select all the items or you can just place check marks by multiple items. You don't have to do all of them:
Now click Edit visibility at the bottom of the screen:
Finally just select Visible to students and then click Save Visibility:
2. You may do it on a case by base basis by finding it in the content area and click the menu to make it visible:
3. You can cake it visible when you create the assignment/test. There is the option at the top of the assignment settings and you can select to make the item visible: