So in the wardrobe there's clothing and then there's items, which are full of really cool outfits, wings, backgrounds and holdables.
How do you unlock the majority of those?

You will have to buy them in the store with diamonds, there is also a giveaway to newbies on i think the 2nd page on unofficial games, that's how I got these items

There are many ways to get cool avatar items.

The items you have to buy with diamonds are the ones with a calendar icon or a diamond icon in the top left hand corner of their box things (the calendar ones only appear in the diamond shop for a very limited time, however).

If you check the trade broker, many avatar items sell for gold quite cheap. The rarer the item, the more expensive it sells for. Wings tend to be quite expensive. (If you are low level, I would not suggest buying at the trade broker unless you have enough gold for items and feeding your pets)

Items with a map icon can be found by exploring the world. Defeating some mobs has a chance to drop items. There are also avatar item vendors in some of the zones (which you unlock in Astryl's Vale)

The final way to get items is through festivals. The icon for these items looks like a yellow sun. Major festivals happen four times a year during spring, autumn, winter, summer. There are also some minor festivities for days like valentines day, etc... You can trade festival currency for avatar items, and you can bash up festival bosses for avatar items.

You can get them in a variety of ways. Some are from the diamond shop, and those have a diamond in the corner, but the majority of the items are gained in other ways.

If there's a little picture of a landscape in the corner, then you gain that item through exploring (usually random drops from enemies).

If there is a little elf girl's face, then it is an item from a forum game (these are difficult to get as they're pretty exclusive to that particular game, that particular year, during that particular festival. They can also be given as prizes when you win a game and are given the chance to choose an avatar item prize).

Ones with a little sun in the corner are from festivals. Some of these are harder to get than others, depending on how they were gained in the festival (bought from the festival store, dropped from regular enemies, dropped from bosses, dropped from special bosses, or found in special mini-games).

Those that have a little calendar on them (kind of just looks like a little white window) are monthly items, which means they were released specially for one month in the diamond shop, then have not been available again (except a special sale on Black Friday weekend). These tend to get expensive because of their limited nature.

The final type that you won't actually see listed in the wardrobe until you actually have one is a custom avatar item. These are created by players from templates given out along with the introduction of a new monthly set. Players recolour the items the way they want, then pay in diamonds to have their item created. They can also buy extras of their item for 100 diamonds each to sell or give away to other players. These are marked with a little paint palette in the corner.

Sorry that was so long XD hope it helped at least a little!

Thank you for all the help! It helped a lot!