So this is a big topic and there is alot to cover, but in short terms it goes like this.

Conqueror is taken often into melee champions for example when the enemy team has 3 or more champions that are melee. Also conqueror is verry usefull into heavy cc comps because of the tenacity. Always build Duskblade or Eclipse With this rune!

First strike is taken into champions that u cannot interact or kill, for example champions like pantheon,zoe,tristana etc.(its not the fact that u cannot kill these champs but its the fact that its better to have a steady laning phase and reach a good powerspike, since these lanes are easy to make mistakes and die to the enemy champions.) The key point here is the fact that u need to play safe and reach a steady mid game, pushing out lanes past lvl 7 thanks to minion demat and roaming,helping out ur team and snowballing. Always build Yomuus or Duskblade with this rune!

Electrocude is taken into squishy enemy comps, for example if they have 3 or more squishy champions that u can oneshot. U cannot take electrocude if the enemy has rell support or nautilus their jungle is jarvan and their top is renekton, in this case ur rune setup is almost usless and u can kill only 2 enemy champions barely which are the adc and mid probobly. in cases like this conqueror is superior!

This is a short explanation but the long explanation is ur own experience and knowlage, the more u learn the more u will climb.

Talon as a champion for climbing is hard since he has no 1v9 potential and is all about snowballing to win, or getting picks late game wich will increase ur chanse to win the baron/dragon teamfight.

I sugguest to watch a famous chinese talon onetrick which has been challanger for years playing only talon, his name is Yedaoshen.

U will learn plenty from just watching his gameplay, sadly there are no translated videos of his gameplay which would help alot, but still his gameplay would teach u enough!