@Senpai-Noticed , Just for the sake of clarity, I am listing your questions at the beginning of my response.
1 What should I do when I don't know what to do?
2 Should I play defensive and wait for a mistake by my opponent or go for a better strategy?
3 Or should I learn an aggressive opening as black?
I will do my best to answer these questions. Most of my time on the first one. That's a big question.
1. This is a question that a lot of inexperienced players struggle with. They know what to do when there is something to do, like take a piece or attack the king, but they are at a loss if their opponent doesn't mess up and start giving away pieces or pawns.
The answer here lies in one word. Strategy. A well known grandmaster once said, "the tactician knows what to do when there is something to do. The strategist knows what to do when there is nothing to do". At every point during a chess game, you should always have a master plan in mind. It is how maturely you formulate this plan, and how you put it into action, that decides how strong a player you are. So, that is the general idea, but as for practical advice, there are several things that you can do in a quiet position. Here are some of them.
a. Improve your worst piece.
b. Limit your opponent's pieces, and or pawn breaks
c. Prepare a pawn break of your own
If you don't quite understand what I mean by one or more of these, just say so, and I can elaborate.
Once you decide on a course of action, (often the hardest part), you must then devise a series of moves that will accomplish the goal. For example, "I want to play c5 to attack the center". Now, you may need to give support to the c5 square first. This is a very basic example, but I hope that you get the idea.
Once you reach a higher level in chess, your opponent will often see these ideas and try to shoot them down, so that the game becomes a duel of ideas, tons of fun!
2. Playing defensive and waiting for your opponent to make a mistake is almost always a terrible idea. At the low level in chess, you can generally count on your opponent messing up eventually, but you will have messed up first if you sit and wait. Once you reach a certain level though, you will find that your opponent has a plan in mind with his moves, and sitting and waiting for a mistake will suddenly result in being utterly unprepared when they finally bring their plan to fruition. There are times when defensive play is necessary, but it should always be with a view to defend against the assault and continue with your masterplan, whatever that may be.
3. An aggressive opening as black is completely your choice. Sicilian, caro-kann, kings pawn, French, are all acceptable. Your opening was not the problem in this game. You just need more of a coherent game plan, and good board vision.
Hope that this helps. As always, any questions are welcome.