Okey, so here are my results:
I have made a variable that was an amount of picked up items.
I have made a function that add 1 to the first variable that was used every time character was running trough the item, picking it up.
There is how in by variable was written a number of picked items.
Then I decided that player has to collect 5 items to win.
I set this in my Character Blueprint. It works basiclly in that way:
If my first variable equals 5 set the boolen variable WIN true (obiously the default setup for it was false).
Than if WIN is true, disable input,wait a few seconds, play a victory sound, wait few seconds more and than open a new level with a text YOU WIN and two buttons: play again, go to menu.
I am not sure if I used a proper language, I am a beginner, but that was my solution.
I thank for all replies 