Should I trade bishop for knight early?

Usually it's better to keep the pin as that may bring you tactical opportunities later. It's also more uncomfortable for your opponent if you keep the pin. By making the capture on f6 you free your opponent's position and help them to develop their Queen.

if you are able to delay a pin on the queen until your opponent has castled kingside then you get some extra compensation by them weakening their king when they push the h pawn. In the example you give, your opponent has the option of castling queenside and using h6 as the basis of a pawn storm or to grab some white squares on the kingside so has lost little with the pawn move.

White shouldn't have put the Bishop there to begin with. Black's game is already better.

I remember when I was new, I read some note about bishop takes knight on f6 was good because it "traded off black's important defender"

Of course it meant that knight was an important defender only in that specific position... but I was new so for a while all my games rushed to make this trade early.

Anyway, like other people said, don't do that

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Not that I disagree with others, but I think it's important to mention that rarely do you want to set your central pawns on one color, then rush to trade the bishop of the opposite color.

And when you do, you'd at least want to double their pawns. In this position black can recapture with the queen.

So yeah, Bg5 in that position is clearly an inexperienced move

Trading dissimilar minor pieces can be very sound, but you must keep in mind the pawn structures which make this advantageous.  In the example above, I'd suggest the exchange is bad.  In addition to the loss of tempo Keto mentions, there's the issue of d3 having been played.  White's pawn structure is therefore weak on the dark squares, making the lack of such a bishop problematic.

In general, you don't trade a bishop for a knight in the opening unless your pawns are mostly on the same color squares as the corresponding opposing bishop.  This means that your remaining bishop will remain strong, while the opponent's 2nd bishop remains weak.  The bishop pair becomes more powerful in open positions, so you will need to be mindful of that as the game goes on.  However, if the pawn structure is favorable, such an exchange can be very good.  The exchange variation of the Ruy Lopez is a good example.

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I hate knights (not mine) I wipe them out ASAP