Some members have already correctly answered you let me try to elaborate on that.
imho 3 things and they are Method, Mindset, Money management.
1) Method: You need a method with an edge for example say you have a coin which is loaded so that one side comes up 60% of the time instead of 50% of the time for a uniformly made coin. It simply means over time the chances of a particular outcome are higher than a random distribution. This constitutes an edge.
2) Mindset: Even though you have an edge you must still have the discipline, and tenacity to continue trading in the face of losses knowing over time your edge will kick in and you will be profitable. This really is very hard to do. For example in the above example even though you have 60% chance of one side say heads coming up over tails there are still time when you toss the coin and you will get a string of tails or losing trades. you must have the mental discipline and emotional control to continue in the face of losses knowing you have an edge and that over time it will kick in and make you profitable.
3)Money Management : There are two parts to this (a) keep losses small (b) position size correctly
(a) You must keep your losses small because if you risk too much and when you encounter a string of losses it may deplete your account to such an extent that you don't have any money left for margin to take the next trade so its game over even though you have an edge.
(b) You must position size for optimal growth depending on your personal preference of optimal. For eg
Say you have a fair a coin which comes up 50% on either side but when you win you win Rs2 and when you lose you lose Rs1 this is positive expectancy so after 100 flips you will have won Rs50/-
Lets say you have Rs100 to start with queston is how much should you bet on each flip? say you chose to risk 10% on each flip so if you started with Rs100 and bet 10% and won you would have won Rs20. so your new total is Rs100+profit of Rs10 = Rs120 .. so betting 10% of that would be betting Rs12/- on the next toss of the coin if you won you win Rs24 but if you lose you lose Rs12 so on and so forth
so with the same odds at the end of 100 tosses of the coin
risking 10% per toss you end up with Rs4700/- a return of 4700%
risking 25% per toss you end up with Rs36100/- a return of 36100%
Do does this mean the more your risk the more you return? lets see
risking 40% per toss you end up back with Rs4700/- a return of 4700%
risking 50% per toss you end up with just Rs36/- a loss of 64% !!
So position sizing is critically important as I've demonstrated but its not the only thing, we can conclude the 3 M's method, mindset & mm separate the winning trader from the losing one