Here's what i did 2 yrs ago:
-I gathered all the cards that were lying around the house and put them in binders (if you have any left, that is).
-Now that i have a job (i work in retail) i can buy my own cards and not have to bug my parents about it. Plus since i work in retail the cards are right there, my employee discount also helps.
Tips for what to buy:
-If you want to collect, keep buying a bunch of booster packs, i recommend the ones from Heartgold/soulsilver unless you don't mind the black/white sets which are in abundance right now.
-They also have those random cards like say 25 random cards for 4$ found at toys R us, they include at least 1 rare card. The downside is that they're mostly from diamond/pearl.
-AVOID: those championship decks. Those are decks made of hard to find cards sold for cheaper however the back of the cards are different from regular pokemon cards, making them absolutely worthless.
-You can get various starter decks for 15$ however cards found in starter decks have reduced value. Thy're good as a lump start for your collection though.
You might also live in an area where they have more variety for sale. Keep exploring. Also buy your plastics at the dollar store, they're MUCH cheaper.And that's just how i would do it.