I generally find that you can make matches enjoyable in the following manner:
1)Play quick, accurate and loose. don't make announcements, don't rejoice too much when youre winning/lucky, don't make fun of your opponent when he has bad luck/losses. don't overemphazise the things you do, talking to yourself when you measure/plan might sound infuriating to certain players. Do not brag about your units or tactics, bragging especially is unpopular if nobody asked in the first place. Put the measuing tape as close and flat to the table as you can and give your opponent ample time to see it, that way nobody has to second guess ranges and it will circumvent a lot of possible arguments. don't pick up the dice immediately after rolling them unless they are negative for you (the enemy wants to know how good your roll is, he doesnt care how bad it was). Pay attention to your opponents measuring and dice when they are in use, especially if they dutifully watch you play during your own turn, not being present or not caring about the enemy moves may feel like a slap in the face for some players. Roll properly, don't just drop the dice or use some unique roll technique. If your opponent has the ability to react to an obvious threat but does'nt, like not spreading his models out when they are close to a transport full of templates and plates, remind him, that "oh right" moment can ruin your game so don't let it ruin other´s games. If someone makes a mistake that is easily and immediately reversible, reverse it. If something is not absolutely clear for your opponent but is absolutely clear to you, remind your opponent often. Bring your own dice, plates and ruler. Play quickly rather then meticulously, for example: move the first few models in a squad accurately, then bring in the ones behind more quickly and haphazardly.
The above is really just gamer ettique, many more examples exist that I may forgotten or am too used to. Some of it wont be nessessary for some types of players and is absolutely nessessary for others. The list is long, but really its all just nearly basic human courtesy. Win modestly and loose graciously.
2)don't brag! Basicly reemphazizing this part of gamer ettique. don't talk like your strategy is "so good" or a certain unit is "so powerful" or "just so damn useful". don't emphasizze a units power unless it lives up to it on the table, a Riptide wiping out marines is par-course, firewarriors punching down a demon prince isnt. If either player has something that is overpowered don't talk about it all game long.
3)don't bring what the internet tells you nor the absolutely strongest armies you can possibly assemble (strength needs to be proven through vigorous testing and is meta dependent, a wall of guardsmen is tough as nails against an army of nothing but naked gretchin, so if your local meta is "naked gretchin", a wall of guardsmen is a no-no, bring naked gretchin).
The point above will irk experienced players who do not themselves build from the internet and no self respecting newbie will respect you once they realise what you did. If you know specific individuals who powerplay then don't hesitate to go all out vs those players only. Don't ever tell anyone with a non-powerful army who you just beat with a powerful army "its perfectly legal, you can do it too", it will just exasperate them.
4)Read your opponent. Some people love to chat about op units. Some people like to make the whole battle a jokefest whoever might be winning. Some people hate smiles with a vengeance. Some people are straight faced during a match and all "do you remember when!" -after.
5)Your opinion is set in stone, an invisible stone buried somewhere under the great gas mass of jupiter along with the opinion stones of every single other human in the world. Nobody is ever going to see your opinion stone so don't try to describe it to them in intricate detail unless asked to (not agreeing with you is not the same as asking). If you cannot agree on something make a 50/50 roll on it, if they still disagree get a 3rd party older player or judge or check the net. Note that checking the net is a last resort, play quick and loose! Use your age group as a platform to jump from, land in the average age group of your fellow players, be the best of both worlds.
6)Smile, smell nice, be presentable. Keep your models and gaming devices presentable.
The text wall above is everything I can remember to be a fun player to play against. My 150 strong ork horde has never been unwelcome on any gaming table since I made it a Decade ago. All of the above is dependent on your exact personality and your oppoent's. Sometimes a person just isnt going to like you, the way there probably exist people that you absolutely hate the idea of being within earshot off. I hope it is as interesting a read to you guys as it was an interesting thought practice for me.
In the war against humanity, forethought is key.