Generally speaking, I'd go with a preconstructed deck, if only to get the energy cards you need. take a look at the ones available from the e-sets on up and see if there's anything there you like.
along with purchasing a preconstructed deck, I'd recomend buying as many boosters as you feel comfortable purchasing, 3-5 is a great start. but even a single booster is helpful if you're strapped for cash. Try to buy boosters from the same block (e cards, or ruby/saphire) of cards so that they have a better chance of the pokemon going together.
the next thing to do is look at your new pool of cards and see what you have good evolution lines of and what compliments your preconstructed deck.
next thing to do is change out cards in your preconstructed deck to make it as consistant as possible. for instance,
say your precon deck comes with 2 natu's and 1 xatu. a fun pokemon, but a 2-1 line isnt very good, if you happened to get one more natu and one more xatu in your booster packs, then you could add those two cards and change from the weak 2-1 line to a decent 3-2 line.
the more consistant your deck, the more likely it's going to do what you want.
if you didnt get anything that's really usable for your precon deck in your boosters, thats ok, these cards were made for trading. See if you can trade for cards that you need for your first deck.
other than that, I'd suggest looking at some of the peoples decks here and you'll get a feel for advanced deck creation.
things to keep in mind :
consistancy, (dont do alot of single cards, make sure you have 2-4 of each card you want to use in your deck)
dont go overboard on the pokemon (try to stick to about 20 pokemon [that includes copies of a basic and evolution cards])
dont forget good trainers.
have fun with it!