I assume you have installed a loot filter. See
http://filterblade.xyz/ if you have not. Choose a filter level that suits your speed (at which you kill stuff). When in doubt, choose "regular". Loot filter will cut out a lot of useless stuff out and it will highlight more important and valuable items for you.
When looking at items, you should be looking at their stats. Basic stat priority order:
1) Life -- most important stat in game. Damage is infinite, but life is only available from a very limited number of sources.
2) Resistances so that you are capped. Chaos can be largely ignored, but you will want SOME eventually or chaos damage will just kill you. Just don't leave it at -60%
3) Damage boosting stats
4) Other stats
That's the priority order. Many new players make the mistake of making damage stats the priority and ignore survivability stats. As a result their game experience will be horrible. You will learn to optimize your items better over time, but that 4-point priority list will help you get started.
Currency tab is the best tab IMO, and should be first you get. 1 premium tab so that selling things to other people gets easier. Rest of the tabs are just for making your life easier. Map tab is also pretty neat and helps immensely with map sorting. But you don't have to worry about that until you get to endgame. You CAN play PoE without spending any money on tabs, but IMO currency+1 premium will make your life a lot easier.
If you are not the kind of player who wants to play SSF (Solo Self Found) then you should learn how to trade.
http://poe.trade/ or official site
https://www.pathofexile.com/trade/
The basic idea behind money making is that because PoE loot has a lot of RNG behind it, it's pretty challenging to find the exact best items your character needs. But you WILL get drops that are valuable to other players! So what you can do is you sell the good items to other players, and buy stuff you need with the money. It takes time to learn what is valuable and what is not, but you'll get to it.
Hope this helps you get started.