So you're thinking about jumping into tabletop war games. Great! But where to begin? Let's assume you've chosen a sci-fi game with a huge existing player community and a wide range of different armies published by a mercurial British game company. I think this kit is a decent place to start: Getting Started With Warhammer 40,000.
The book offers a brief overview of the armies in the Warhammer 40,000 ("40K") universe, some good painting tutorials, the basic rules, a walkthrough of how to play, and a lot of other good info. It also feels like a huge advertisement, but that shouldn't be a surprise at all. Games Workshop wants you to buy more Games Workshop plastic crack. That's why you would be interested in buying this in the first place.
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I am of two minds on the Primaris Space Marine miniature. I do like the look of the basic Primaris Marine model. I don't like the background story of the Primaris Marines. I do like the detail and design of this press-fit mini. I don't like how the standard Space Marine has been relegated to the sidelines despite being one of the most iconic soldiers of the setting. I'll probably assemble and paint this guy eventually, but I have zero interest in collecting more.
£5/$8 is a good price point if you want to dip your toes in the water. If you know someone who already plays, they can probably help you paint the mini so you can see whether you like that side of the hobby or not. It's a good way to see whether it's for you.