First up, you need to accept that the PT 'year' starts afresh with each MLB season. You are therefore way behind the curve starting when you have, and in all honesty you are best off just using the rest of this year to gain experience in the game and how it works so you can hit the ground running next spring.
Secondly, as in real life anybody can beat anybody, not least in the playoffs. The better the team, the more the odds are in your favor, but there are no guarantees. We've all been there, on both sides of a 'shock' result
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So..
The only way you will advance is by getting better players, and the only ways to do that are buying them from the auction, drawing them from packs, or (rarely) winning them in tourneys. In practice, unless you have real money to waste/burn, that means winning packs in tourneys (watching the live streams throws you a few as well). Keep what you can use, sell what you can't, both in the team and for collections. Concentrate on building up iron and bronze tourney teams initially, learn from the teams that beat you and you'll soon start notching up player packs. Use PP to buy players, not packs. You can gain PP from events, such as hitting a grand slam, all-star selections, and so on but look on these as a nice bonus, not your prime source of 'income'.
You can do a lot tactically, but not work miracles. It's a loop really, early tactical choices are dictated by the players and skill-sets you have, but as you progress you will find yourself more and more buying players to fit your tactics. As a very broad guide, look for hitters with good contact and avoid 'k's (strike-outs), and pitchers with good movement. Other skills are important but not, generally, AS important. Look at player skills carefully, but use card 'value' as a general guide only. Not all cards with the same value are created equal.
And a final tip, don't neglect to customize your ballpark to how YOUR team plays. It can give a small, but significant, edge in your home games. The game tells you how the average number of HRs, doubles and triples changes with ballpark dimensions.
P.S. As to 'new to baseball', there's not a lot you can do other than watch and study the game. The MLB even puts one game out live most days for free on Youtube during the regular season and their subscription is good value - get one for the play-offs if nothing else. Read at least one good book on baseball statistics; there are several available.