Starting off in Asana can be overwhelming, especially because it’s so versatile. The versatility is what you will love most about it though, so hang in there. Tasks are separate from projects so they don’t delete if you delete a project. You can create a task that either lives alone or as a project, but to be honest I like to attach everything to a project to I can easily organize my projects. I even have a project for my random tasks to live. A team is a group of people that have access to a set of projects or tasks. There are some great courses in the Asana Academy that might help you, especially the Learn the Asana Basics and the Ways to Use Asana.
One thing you will want to determine is what you want clients to see? Will they have random tasks across multiple projects or will they have multiple tasks within a project? I believe if you give someone access to the project they will see the whole project, including what they are assigned. There are some posts on what collaborators and private guests see when you give access. @Bastien_Siebman had a great one the other day Who sees whom, a guide to guest visibility with Batman & Robin.