I am a player rated 1400.
Whtat should I do to improve myself?
Should I learn more about openings or endgames?
I know that if I will study tactic problems or learn from strategical games depends on my style.
But, if you are a nice tactical player you can find your oponent's mistakes and reach a better position.
By the other hand, if you are a nice strategical player you can take advantage of the open files etc. and reach again a winning position.
So, you think that I have better study more the openings or the endgames?
Chess is more on strategy, so you must focus on how to play for a strategy like positional chess. Of course, this requires the back up of tactics to support your strategy.
You need to study on both cases, opening and endgame. But it's better for you to start on opening.
Inb4 'spend time on tactics and endgame, don't waste time on openings'.
Will depend on your strengths and weaknesses as to what proportion of each area you need to study, someone would need to look at your games.
If chess were a cage fight, tactical shots are your punches, kicks, and combinations, while positional chess is your angles, ground game, locks, and ring control. Both are necessary for the beginner. Which you choose first depends on where your current weaknesses lie, as in, why do you think you are losing the games you are losing currently, and which can you as an individual benefit the most from?
I have a theory that a good position allows for more tactical shot opportunities, such that one who establishes "side control" on the mat has a greater chance at launching an effective "ground and pound". (I watch a lot of MMA)
Honestly though, for me, tactics aren't really studied, but practiced. I think you can do well just getting an introduction to certain tactical motifs (pins, deflections, skewers / x-rays, attraction, forks, discoveries, etc), and do daily tactics puzzles while studying positional chess (your sillman or nimzo books). On a rudimentary level, consider best move as a combination of positional gains and tactical awareness.
Learning opening books too early is like learning about the opponent's fighting style before mastering your own. Beginners should generally avoid this, unless they are gifted with photographic memory, in which case the world is your oyster. Opening principles should suffice at the moment until you've built enough positional knowledge to comprehend what's the strength and weaknesses of each opening as they are introduced and learned.
Interestingly enough, endgames are more important than openings because the tactical considerations employed are practically applicable throughout a larger portion of the game, as opposed to openings, which apply for 10 moves at most, and with much longer study time and preparation because of all the variations.
From those considerations, you can sort of guess which I prioritize the most (positioning and tactics, then endgames before openings). Doesn't mean it's the "best model" for learning chess, but that's what I, another amateur am doing at the moment.