Help getting started

I used to collect the cards when I was younger, but I never really got into competitive battling, so all I really have are heaps of gen 1 basic pokemon and energy. Woopee.
I'd really like to get into competitive card battling, so I figured here would be a good place to start. How much can I expect building a good deck to cost? Also, I was watching some videos on Youtube of top level players and it seemed like they all used novelution and legendary pokemon, and I was just wondering if that's a usual restriction if you want to be successful on a higher level, or if it's just coincidence and luck of the draw for cards available that year? I'm probably gonna wait until this year's season is over before I really get into building a deck, because I don't want to spend the time and money to have it become irrelevant next year, but I would like to get a solid basis. Any help is appreciated!

First thing that comes to mind is that, generally, a set will have two years of shelf-life where it'll be legal in competitive play, so if you were to buy any of the 4 boosters on store shelves right now, as well as Dark Explorers boosters, you'd likely be good to play using those cards until the Summer 2014 format rotation, which generally happens at the end of August each year, rotating out a single year's worth of sets, which is generally 4 full sets (January, May, August, and November are the typical release months).

As for the non-evolved and Legendary Pokemon thing, that's just down to how the format is right now, and it's mostly because of a card called Pokemon Catcher, which, unfortunately for evolution decks (those that use Pokemon that evolve), will also be play legal until 2014. Basically, the big 4 deck archetypes right now are called RayEels, Blastoise/Keldeo, Big Basics, and the Darkrai variants. There are a couple second-tier decks that can beat any of the top tier decks if those top tier decks aren't prepared for them, though, and those are the Klinklang and Garbodor deck archetypes, which specialize in stall and slowing the game down considerably.

Personally, I run something else entirely because I'm looking at something that's going to be different from the mainstream decks, so I don't expect a whole lot of success.

A list of modified format legal sets is here: https://www.pokecommunity.com/showthread.php?t=289800

A guide to the basics of deckbuilding is here: https://www.pokecommunity.com/showthread.php?t=275378

Some things to note: Pokemon Catcher will be an essential card to have even if you start playing after the next format rotation, which will probably happen in August and remove just Black & White, Emerging Powers, Noble Victories, and Next Destinies from the format, because Catcher, originally printed in Emerging Powers, was reprinted in Dark Explorers as a secret rare. Unfortunately, a single copy of even the Emerging Powers (Catcher having a rarity of uncommon in this set) print can run as high as $20. Other trainers that will be cheaper but still essential and worth a look will be Professor Juniper, N, Skyla, and Cheren, at a minimum, and then you'll also need to look at Pokemon fetching trainers like Great Ball and Ultra Ball.

EDIT: Don't forget to just look around and see what cards have effects you like! The most important thing is to have fun!