Warhammer 40,000 is a futuristic fantacy game where you pick a faction with rich lore and a unique style and wage war in the name of a supreme leader, some other entity, for the fun of it or even just for food *nom nom nom.
The easiest way I can explain Warhammer 40k as a game is it is like complicated chess.
In chess each type of piece has a use, a movement and a way of attack. There is also a specific goal to achieve by using these pieces.
The same is with 40k but like I said, its more complicated...way more complicated. Instead of a grid the game board uses open movement, mesured by a ruler and dice are used to determine decision outcomes. Each turn is divided to make following rules easier.
Every faction is the same in the sense that they are made up of units that have a particular use and can work together to create a desired outcome. However one faction will not play the same as another and will achieve the outome differently. This is known as a factions "play style".
The best and easiest way to decide on a faction is to look over the warhammer website and chose models that appeal to you as you will be spending some time building and painting them.
The cheapest way to start playing is with the Combat Patrol box sets that are small starter armys, ready to go (but you still need to build them).
The game its self is broken down in to 5 rounds and each round consists of two turns, yours and your opponents.
Further more a turn is split in to phases, -Command phase
where you can organise specific rules and determine score.
-Movement phase
This is when you maneuver units to score points or prevent your opponent scoring.
-Shooting phase
At this stage you attack with ranged combat at selected target and perform point scoring actions.
-Charge phase
where you can attempt to move models in to close combat.
-Fight phase
This is the final part of each turn where you attack and defend in close combat.
Games can last several hours depending on the size of your army so it is usually suggested to use whats in the combat patrol box while you are getting to grips with the rules. This way games will progress quicker as you learn.
There is devided opinion on etiquette for what condition models are in to play. The general rule is "table ready" which means a minimum of three colours and some basing done however the majority of the community are generally happy aslong as models are built and identifiable. We all want different things from the hobby and maybe painting isnt for you. Just speak with your opponent before the game if you need.
Thats really it in a nutshell. There is obviously alot I have left out regarding rules etc however these are all free to download here at the bottom of the page along with "how to play" videos
https://warhammer40000.com/getting-started/
