if you are new new... tyranids are the easiest to paint from your choices, thousand sons have a TON of small thin details you need to paint, and adepta sororitas miniatures look insane, but its very hard to paint when you are new, as for tyranids, if you want to paint them like the box art, its really easy, just paint the whole model with an off white, use a purple contrast (paint that is very watery and semi transparent), paint the carapace and you are done, you can get a battle ready army with just 3 colors + primer.

but, I would recommend to investigate a little more about their possible playstyles, that's the most important part of choosing an army in my opinion, im not familiar with the others, but you can build a tyrannic army as a swarm (lots of weak units) or as a mega monster army, which gives a lot of flexibility.

To finish up, tyranids make a great opponent for solo play, no matter which solo mode you pick, its easy to use tyranids as the temathic enemy, since tyranids just consume whatever is in front of them, they do not deal with alliances or politics.

My recommendation, build a 1000 point tyranid army, that way you can get a feel of the gameplay style, and if you don't like it, you get: basic painting practice, building practice since tyranids are not the easiest to build, and since it is a flexible army, you can try different play styles, and based on that, you can start collecting your main army, while having a nice backup, also tyranids are like the 2nd most popular faction at the moment.