Welcome to the wonderfully addictive world of MTG!
My advice: If your local store offers draft, draft for a while.
1.) It has a low up front cost
2.) You aren't at a disadvantage for having a fledgling collection
3.) You learn to evaluate cards, and synergies
4.) You learn what style you like to play, aggro, midrange, control, etc.
5.) You build a collection of standard cards that you can:
a) Use to build decks, or
b) Trade into cards to build decks
Do this for 3-6 months, and you will both become a solid player, and you will have a respectable enough collection that you can start trading into whatever constructed deck you want to go for.
If your store doesn't offer draft, and/or you really want to get into playing constructed:
1.) Don't get frustrated by losses. You will be at a disadvantage both in terms of experience, and available card strength for a while. Play to learn, and the wins will come in time.
2.) I would advise against just buying packs. They give you a more general collection, so you can do a lot of things, and you can trade, but you won't likely do any of those things well, and it will be awhile before you have sufficient cards to trade for the more powerful/valuable ones you need. Packs are typically a losing proposition value wise. You will spend a lot of money on packs before you get the cards to build a competitive deck. (This is why I advocate drafting. All the advantages of buying packs as far as building a generalized collection, but you get competitive experience in the process, and given there will usually be some prize-pool, your expected value is considerably higher than just buying packs.)
3.) Try to invest minimally until you know what kind of deck you want to play, and you know that that deck will be at least reasonably competitive. There's no worse feeling than dropping some money to build a deck that doesn't turn out to work well.
4.) Once you do have an idea of what you want to play, buy the cards. It is a much bigger up front cost, and that can be a big turn off, but in the long run you save so much time and money vs. opening packs to try and get or trade for what you need.
Other more generalized advice:
1.) Ask for advice (you're already doing this one, great!) Most stores have a community that is supportive and encouraging. There will be lots of players that will be willing to offer you help, or critique mistakes, or show you options you missed. Embrace those critiques and learn from them. The game is crazy complicated. I've been playing most of my life, and am still learning new interactions, sequences, rules, strategies, etc.
2.) Read. Watch. Study. There are lots of sites out there that have free articles offering deck ideas, or tips on how to play. I read a lot of the Starcity Games articles daily. They also have videos where one of their pros will take a deck through some matches online. (Channel Fireball does the same.) Watching these, and listening to the pros explain their thought process can be a fantastic learning experience.
3.) Playtest. So you have a deck idea, but don't have the cards for it. Proxy cards. If you play with sleeves (which you should, to protect your investment) you can simply put a slip of paper into the sleeve with any other card that says what that card is representing. If not, get a stack of basic lands and you can use sharpie to write on the front. This allows you to test how the deck will work, what cards are and aren't good in it, and allow you to tweak before investing in the cards for it. Obviously you can't play in any competitive events like this, but if you have friends that play, getting in casual practice games like this is great. Also, most stores have people that hang out well before the start time for events. Go, hang out, and ask if anyone wants to play a game or two to help you test your deck idea out.
4.) Lastly, and certainly not least importantly, have fun. This hobby is different things to different people. Some love the competitive aspect. Others the collecting, or the trading, or the financial. Others love being creative, and finding fringe deck ideas that may not quite be competitive, but are fun, and innovative. Figure out what most drives you with the hobby, and run with it.
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