Can anyone give me a guide on how to trade item in dota 2? thanks in advance

First of all, you should have friends in Steam

Go to DotA 2 game, in the lower right part, you will see some friends (some of them is online, offline or in DotA2 or maybe other games)
left click one of them (choose one of your friend who also have DotA 2)
there is a selection called "trade items"
and you choose which game items you wanted to trade and just follow trough

Hope you understand this..
http://steamcommunity.com/id/indraguna/

or you can trade with not friends by viewing theris Steam profile and clicking 'trade'. There are a lot of "Dota 2 trade" groups where you can get nice stuff and there's no need to add everyone as a friend

Trade can be done out of dota as well with friends; with the friends window, right click on a fellow friend you'd like to trade with and select "Invite to Trade"

Steam keeps giving me errors when trading.

1st Go and see item quality
There is a few different qualities
Common - price is lowest
Uncommon - a little bit better than common
Rare - a lot better than common
Mythical - best quality
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On the Market you can find Genuine items,Immortal and stuff like that,i personally dont like them,reason is simple,too expensive