the best move is to move my f pawn?!

in this game 

https://www.chess.com/analysis/game/live/24628916723?tab=analysis

the computer says that move 6 I should have moved my f pawn.  But isn't moving the f pawn a big no no?   I guess 'cos it opens a diagonal attack on the king.. and  does it mess up pawns in any kingside castling?

I can't see how that scenario plays out according to the computer e.g. is king-side castling then a no no? 

And castling aside.. It opens up a diagonal attack.. so i'd have thought that if moving the f pawn was a good idea, then it'd suggest I messed up before it. But all my moves it says were book moves before that.

Typically moving your f pawn isn't advisable in the opening, but black has to relieve the pressure on f7 in this particular scenario in order to survive. If black simply tries to kick the knight with 6. h6, white will sacrifice on f7 with Nxf7, and after kxf7, qf3+ with a fork on the knight on d5 and the king on f7. 6. f6 in this particular scenario relieves the pressure on f7 and kicks the knight, and after the knight retreats to f3, black can play nb6 and avoid any trouble

For future reference, this is how you should play against 4. ng5

This is the computer being a computer. 
Take another look, and try queen takes knight.  This wakes up the computer and its like "oh yea.  Thats pretty good" (beavis voice).   The F pawn is an immediate gain for a longer term positional loss, but the computer's depth is not picking up on that, and ... for all I know, the longer term weakness is no big deal due to some strong attack it has cooked up.  But if that were my game, queen takes is safe and effective. 

The opponent will play BXN and you have a juicy B-h3 threaten mate and looking to trade for his rook.  Its not available right off, due to that bishop that just took the knight, but it may be worth a shot in a few moves.  More likely he will play d pawn to attack queen and you will have to retreat it, but you never know.  There are choices for both sides here.

sure of course @Nomad1004  rules aren't set in stone, so part of my question is kind of asking when it's right to move the f pawn! e.g. I agree with @Jonin about taking the knight with my queen being the normal move and one I didn't notice. But suppose white's knight was defended by by his C bishop)as it often would be had he pushed his C pawn. So if taking his knight wasn't an option.. Then maybe I should move the f pawn then..so i'm wondering if there are ideas about what cases it makes sense to and what cases it's a big no no.

This is maybe a situation where the answer is that the guidelines don't apply. You are confronted with a specific attack called the Fried Liver. It's a tricky line and actually played occasionally at the highest level. It is a very difficult opening to defend against when your calculating on the spot. There are moves that are good and solid, but there are also very natural moves that will immediately put you in a near mate position. This is where opening theory comes in. Simply knowing your lines.

So, I guess the rule is 'don't play the f-pawn unless your opponent plays the Fried Liver'. Though you might expand upon that later. 

Btw, it's a fun and interesting opening to study. Make sure to look up Traxler counter attack as well.

The rule never to move the f-pawn because it "weakens" the position is for the most part, correct. However, there are always situations where it is a better move than any other move. You should usually only move the f-pawn when

1. There are no decisive threats that could be posed on your king in the near future OR if you have a long lasting defense against any threats

2. It wins material or something without blundering

3. Your king does not get exposed and the move defends or has an idea

Of course, there are always exceptions, but usually do not make the move when it has no purpose.