I don't think I'm too good at deciding when to engage or not. I often find myself having waited too long for favorable conditions, or engaging when I shouldn't have. This makes my KD suffer pretty badly. Any general guidelines on when to engage?

If you are the tank, then you need to engage when your team is in position. If you're not the tank, then wait for the tank to engage before you run headlong into the enemy team.

General rule of thumb on engaging is to make sure your team is ready for it. If they are, then you wait for the enemy to be out of position, i.e their tank is in the back of the team, their carry is too close to you, 1 or more of them are not in the area you plan on engaging, etc. If there's trouble brewing in mid-lane and someone on their team is pushing bot, then it should be a 5v4, which means your team should be able to win the ensuing team fight.

Having knowledge of where the enemy team members are on the map is extremely helpful in making decisions like this. Having just one or two wards in the enemy jungle or the river in high traffic spots can let you see if there are any stragglers too far away to help if you initiate a 1v1 or small team skirmish. This can also help you see if anyone is trying to flank your team during a fight to get at the ranged squishy champs in back. Also, get in the habit of paying attention to when people use their summoner spells. If you notice that the enemy you want to kill has used up their flash recently but they're still being aggressive, you can take advantage of their lack of escape options by target pinging them to your team and letting them know their flash is down so you all can lock them down with cc to guarantee a kill. I hope that helps.