I'm having trouble with clothes and hair I buy.

[Jadnia]

I'm new to SL, so I need a little help w/ all the technical stuff. Some of the items I've perchased on the website, like hair( such as https://marketplace.secondlife.com/p/MrS-Perfect-long-straight-hair-Black/646311)  and cosplay items turn out to be blocky, advertisement thingys that stick out my avatar. Can anyone tell me why that is?

[Peewee Musytari]

Sounds like you haven`t unpacked them. Most items you will buy will be packed in a box which can be any shape or size or look like a billboard.

Drag it from your inventory to the ground, right-click and OPEN. Once its loaded, click "Copy contents to inventory".

You should then find a folder in your inventory with the items to wear.

Welcome to SL

[koto Nizna]

Here's Torley's video tutorial------->

Apparently the viewer he is using in this video is the original viewer.

[Rolig Loon]

You will discover that you are not allowed to drag a box to the ground in most places in SL.  There are two ways around the problem.  One way is to find a sandbox where you are allowed to rez items.  You can do that by opening your Search function, selecting the "Places" tab, and typing in the keyword sandbox.  Another way to open a box is to attach it to yourself instead.  You have already done that simply by wearing the box.  Once it is attached, you should be able to select the box by right clicking on it, and then select "Open" to send its contents to your inventory. When you are finished, just detach the box again.

[Lyra Blackthorne]

If you are not wearing the "box" as some suggested it might be you are wearing a 'demo" vs an actual item.  If you buy a demo like hair it will usually have a big sign sticking out of your head with name of shop who made the hair.  These are made so you can see how the item looks before buying it.

[Void Singer]

"open" isn't available for worn items, "edit" --> contents is the only way to get at attached contents

[Rolig Loon]

Ooop. Right you are. I regularly open boxes as attachments because it saves me the trouble of going home again to do it, but I misspoke. You do need to do it with Edit >> Contents and then drag the items to your inventory. "Open" doesn't work.