I myself, are more interested in the collection of Pokemon Tcg rather the playing of it however I have had a reasonable start from having collected these cards from the day they came out. The downside with that is they are all played. Since restarting collecting the cards rather than playing them, approx 5 years ago, I have amassed a collection of reasonably excellent condition cards, some mint straight from boosters.
If you choose to pursue the classic sets such as base set 1 and 2, jungle, fossil, rocket, gym heroes etc, the neo series.. They will cost. Also if you choose to get boosters to start off your collection there is an unfortunate culture of pack weighing certainly on eBay which can result in you purchasing a booster that is nearly always containing a non-shiny rare.
However purchasing the cards you want individually will ensure that you get the cards you want, just unfortunately not always in the condition described as.
Perhaps the best way to start is to bring together a nice collection of recent Pokemon sets like X and y. You can get boosters and tins from most superstores and is highly unlikely any will be weighed. There are expansions released on a regular basis and will be interesting to collect. Unfortunately with boosters you will keep getting the same few cards from them as the commons really are common and you may find yourself opening loads of boosters just for one or two rares you might be missing.
I've been getting a folder together of X and y and all the expansions within and have set them out in numerical order and it's frustrating the number of blanks I have in my folder . Beware it is a very addictive hobby but one that will be worth something in the foreseeable future.
Theme decks for any set are ok but you end up with a high number of the same card.
Personally I enjoy collecting cards from the NEO series. They are the last set from the original series and as a result of that the highest quality I find. Issue is that neo destiny for example is one of the most sought after sets in the whole of Pokemon tcg and boosters are nearly extinct.
If you have the money, booster boxes contain 36 booster packs and can sometimes finish a set straight away but are extremely expensive certainly earlier sets.
There is much out there to choose from
Good luck