Aye Space Marines are easy to win with....it's not like all your opponent's army lists and tactics are tailored to beat Space Marines, or the vast majority of their games and so experience is against Space Marines... :lol:
Seriously though, they are easy to play, but not that easy to win with, unless your opponents are at the same starting level as you.
Marines are probably a good starter army as they are without the billions of special rules that you need to learn for something like Eldar and Chaos and are pretty rugged and fairly forgiving for the newer gamer unlike armies Guard and DE.
Obviously as you get better with the army you do find out that everyone tools against them as Jeridian says, but for a kid they're probably a good one.
To keep up your Interest and to help motivate yourself when It comes down to painting pick something that you like the look of, most armies have different variables and style options so stick to what looks cool and go from there.
From a personal point of view Marines are fairly simple to pick up and play and given the various chapters and trait options you can always adjust the style of the army without having to fork out big bucks.
Mech tau may be a winable lis,t but they;re certainly not for the newbies. Properly executed FoF tactics require a very good eye for ranges and use of terrain, and just keeping your J/S/J units where they can line up good shots while maintaining the ability to jump into cover is a skill as well. I'm not saying you have to be a tactical genius to work them, but a mech tau list generally requires a degree of experience with the rules and tactics.
Space marines are you best bet. Chaos are a bit more advanced, and rules conflicts between old and new rules can be a pain. 'Crons are pretty simple too. Few unbits, high points so few models, very tough and forgiving.
Pick the one you like the look of. Because, you will end up painting them if you want to game, and at the end of the day, it doesn't matter if your space marines kick ass on the table. If you hate painting them, then you're in for a very long haul...
If you absolutely must have a recomendation, then go with necrons. They're pretty much idiot proof. You point your shiny skeleton soldiers in the general direction of the enemy, and pull the trigger. And then drop a monolith in the middle of the board, and cause pandemonium on a global scale. There, enemy annihalated. *shakes head sarcastically* A good newbie army anyway...
Most likely Chaos Marines. You'll get more for your $, as most units cost a ton with decent wargear. Start getting into Cult and it's more points for less $ on average. Death Guard are extremely tough, and World Eaters are lethal when close, then Iron Warriors are key at sitting back and annihilating the enemy. The others are more specialized and involve some veteran capacity. Off of Chaos, any beefy marine army will do you well, or even a Eldar army of someone gives you a little insight into a proper Eldar list.
This has been said before but pick an army that you like the look of. You'll be spending much time painting it and I don't think you, or anybody, would want to spend time painting something they didn't like.
From my very very small amount of experience, I would suggest you go with Space Marines, they are easy to battle with, have good saves and have a 3+ to hit if I remember correctly. Although you should probably take a more experienced person's advice over mine because I'm also a fairly new to 40k.
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