Invoker has a HUGE variety of skill and item builds that can be interchanged and swapped to for every situation, so you have to understand as the Invoker player when you can just sit back, farm a Midas and Sun Strike, and when you have to help make space for your team.
The skill that seperates good Invoker players from great Invoker players is the ability to know what and when to build in every situation, this hero does not have a set item and skill build so don't take the build I added at the top of this guide the absolute best way to build Invoker, but I will explain how to know what to build.
Invoker has so many items and options he can build for basically anything you could think of, all you have to do as the player is know what to build against what and when. If you were say versus an early aggresion line up, you might want some sort of escape and early lockdown/ damage to counter their aggresion, for that you could build Eul's Scepter or Force Staff with an orchid, and get an early point in Wex or just go Quas Wex.
Maybe your team is relying on you to take the farming space they aren't, then you may just want to sit back, farm a Hand of Midas for the extra gold and EXP with a Force Staff for some escape, and a Necro Book 3 for some push ability while maxing your exort without Wex for better farming potential.
Those two examples are just a few of the many styles you can play Invoker in, but what you need to remember is the uses your items serve.
Orchid Malevolence- Good silence with a good bonus to your INT and attack speed
Hand of Midas- Very good EXP and gold boost with some more attack speed
Eul's Sceptre of Divinity- Amazing activateable for basically a 2 second Disruption, good movement speed and INT regeneration
Force Staff- A good little boost to INT and great method of escape for no Wex builds
Shiva's Guard- Great boost to Armour, with a AoE Attack slow and reasonable AoE Magic Nuke
Scythe of Vyse- Very nice boost to INT with some Regeneration, and a AMAZING Hex Activateable
Eye of Skadi- Amazing boost to all Stats and very nice UAM slow for your Attacks
Necronomicon 3- Great activatable ability to spawn two little minions that are great for split-pushing, also a good little boost to STR and INT
Invoker is almost the slowest Hero in the game being beat by maybe two or three other Heroes, making him very susceptible to ganks, what makes it worse is that his starting STR and Armour are not better by any stretch, so you need to know how to play against certain Heroes in the lane and how to avoid ganks at all costs.
You will need to be aware of the map at all times and check your ward spots to spot incoming ganks and move accordingly, now if you are going Quas Wex you may not be so susceptible being faster than normal and having the option to Ghost Walk away, but keep in mind that Invoker is one of the squishiest Heroes in the game and if you get stunned for over 2 seconds you're probably going to end up dying.
You may be asking: What heroes are a bad matchup for Invoker and how should I play against them? Well in general what will kill an Invoker is a lot of burst Damage whether physical or Magic, so Heroes like Queen of Pain, Skywrath Mage and the like. Now since you can't really avoid laning against those Heroes without some team coordination you will need to build so that you don't feed your lane, so buying things like Bottle and early Boots of Speed and Ring of Basilius will help your survivability a lot and keep your health up (hopefully). But sometimes those things just aren't enough and you still need help, what you can do then is possibly ask one of your Supports for a gank on your lane, or if they aren't willing you could gank or get kills with Sun Strikes if you are leveling Exort.
Invoke is Invoker's bread and butter, this ability makes Invoker the Spell cranking powerhouse he is, but at early levels of the ability it has quite a long cooldown which can create quite a problem if a fight were to happen and you didn't have any abilities to help your team with.
In the very early game you are going to have one or two levels in Invoke, so knowing what abilities you will most likely be using in the next 20-40 seconds while Invoke's cooldown is active is very important. Say you might be about to help kill and enemy, then you most likely won't want to have only Forged Spirits and Cold Snap available to you instead of Sun Strike or any Wex/ Quas abilities. But if you plan on farming solo for a while, then having maybe Alacrity or Forged Spirits Invoked will be OK.
All I've been talking about in this guide is how to be an amazing Invoker player, but what I didn't mention is to just have fun, Invoker is probably my favorite Hero because of just how much fun I have playing him and if you are enjoying yourself then you are more likely to want to improve as him, so just have fun Eul's comboing people to oblivion ( ^Ï�^ )