The best way to enter this hobby without spending a fortune is to look for second hand armies. This hobby isn't for everyone the game mechanics aren't that good, the balance between the armies is nearly non existent, the models are expensive and painting your army requires more spare time than most of us have so there are a lot of players who give up on it after a while and are willing to sell their army to your for half price.
Expect to have a really nice starter army for 100 - 200 $
I know this is still a lot, so it might be best to do some research of your own, and take your time for it. Watch battle reports, walk into a store where they play games, and read up on the fluff on
http://www.lexicanum.com/ and the tactics on 1d4 chan
https://1d4chan.org/wiki/Category:Warhammer_40000_Tactics Whatever you do please do not buy an army because some forum user told you so.
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Vankraken wrote: My 8 year old son has some Necrons that he plays with and despite his lack of tactical/strategic understanding he has fun despite making a tons of mistakes because they are super durable and easy to use. He can get by with using a lot of Warriors and can shot at whatever and its gonna be at least semi effective.
This is the exception to what I told you. If you are looking for an army to play if you are ~8 there are good and bad beginner armies. Necrons might be just one of the best beginner armies out there.
But if you are able to pass a basic math or any other science test aimed at 14 year olds you shouldn't have any problem with understanding the tactics of all the other armies really.