Your profile stats say your rating is 253, with maximum of 409. Not sure why you talk abt 600. As for openings, they're not as important – in many games opponent will try to sidestep to something different (say central countergambit), so better know general opening theory (control center w pawns and/or pieces; develop pieces; king safety) and try to get decent midgame. Study favorite opening mainly so if opponent plays by your rules, you have advantage. Oh, and stop playing blitz until you're at least 1000.
Which opening should i learn after Italian opening as a beginner ?

You will definitely need more openings as you get higher in ranking levels!
Your supposed to have a minimum of:
- 1 opening for white
- 2 openings for black
Only problem is your below ranking 1,000 which makes things tricky.
You need a very simple opening which doesn’t require a lot of moves because a lot of standard openings will not be played in lower levels due to inexperienced players deviating to much.
You need novice openings!
- Small Center Opening System can be a good option for Black.
As I started playing chess on chess.com, I learned the Italian Opening. For a few weeks, I was playing very well; my elo rating went to 600, but after some time, as my rating got higher, I started losing matches. I think I should learn other openings, so which opening should I learn as a beginner?