Unless you are dealing with a completely blank slate (i.e., a new home or an unfinished basement that also happens to be empty), you likely have some cleaning up to do. You’ll need a space to toss items that are not salvageable, so order a construction debris bin from your local sanitation department.
Then, enlist the help of your household members and start carrying everything out. And by everything, we mean everything (boxes, furniture, appliances, trash, etc.). The last thing you need is to be tripping over unnecessary items while working on your basement.
The level of demolition will range in intensity from simply removing clutter to a full gut which includes tearing out anything that is not structurally important.