When you arrive in Second Life, your avatar is dressed in Linden basic clothing, unless you have signed up to one of the commercial orientation experiences which sometimes provide their own clothing and skins.
What you need to know is that some base elements of your appearance can never be removed, they can only be replaced. Other items can be removed and your avatar can survive without them.
So, you will always need a shape, which controls your avatar's size and bodyshape, you will always need a skin, which controls your avatar's outer layer, and you will always need base hair and eyes.
You can also have items of clothing, tattoes, shoes, attachment shoes, hair, and a multitude of other attachments which can give you jewellery, gadgets or anatomical parts, furry or human.
When you start out, you will probably find that the elements of your appearance are in a folder, but they may not be. My advice to you is that before you start to mess around with your appearance, you make a copy of your current appearance.
It's easy to do this. Right click your avatar, choose appearance. The appearance window will open in front of you, and your avatar will take a special stance that tells other people you are editing your appearance.
At the bottom of the window you will see a button "make outfit", if you click that button a window opens which shows you the items you are currently wearing and offers you tick boxes, some of which are already filled in.
Along the left hand side you will see boxes which aren't checked, for skin, shape, eyes and hair. Check everything you are wearing, including these, and rename the folder using the space provided near the top of the window. I tend to date my avatars, and give a quick indication of the clothing worn.
What will happen is that everything copyable will copy into the new folder. Everything which isn't copyable will migrate to the new folder. This can be a pain if you aren't aware of it. For example, when you start buying outfits, and find that you have saved your avatar and have consequently lost the top from that dress or the bottom from that jumpsuit.
But for now, if you are wearing all copyable stuff, you should now find you have a copy in a new folder.
You should be aware that if you make changes, say to your shape, and save them, the current copy of that shape will now be as you changed it. You will need to go back and find the original to be able to revert back.
If you think of your appearance as being a set of documents in a folder, and altering them as being exactly like changing a word processed document, you will understand... you can only go back if you saved the last version.
The elements of your appearance are governed by permissions in SL, like everything else. Linden stock items are usually set to allow you to modify, copy and transfer them. You should be aware that many of the commercial versions of these things do not. Some will allow you to copy them, but not to transfer them to another avatar. Others will allow transfer, but you can only have one copy and transferring the item to another avatar will mean that you lose your copy of the item.