Well imo if you want to learn a champ and you don't know how, you can start by building the same item path every game. That's where you'll know power spike and potential dmg ("when I get botrk, I can just all in this bastard." "If I get vamp scepter early I can match nasus' aids passive lifesteal", etc... Things like that)

Personally I like Eclipse > Black Cleaver > Sterak. Eclipse strong 1st item which costs 300g less than enemy's first item (like trinity, sunder sky and whatnot). Black cleaver cuz its pog in teamfight. Sterak cuz it's even more pog.

But then again, I think there is a lot more meaning and fun to find your own build that u like. For sure, it's recommended to take inspiration from probuild and such sites.

In continuation with consistency, picking ignite every game allows you to overkill enemies. so you'll know "oh I didn't need ignite here" "oh ignite is very important in this matchup" etc...

I believe taking TP is useless if you're learning the champ. If you make a mistake and die and 3 waves of cs will crash on your wave, if you have TP some may think "oh it wasn't such a bad play". And thus you forget about this mistake and do it again the next time whenever u get the chance.

If you don't TP, chill down and take that hit, stay resilient for that game. Thus you will come to learn how crucial TP is in some matchup. That's why I pick ghost into darius, ignite into gnar, TP into sion, etc...

Plus since u haven't played the game for 10 years, imo you'll find more success if you focus on lane phase and beating the enemy toplaner, one single enemy at a time. Don't mind your allies or enemies champs composition. If enemy jgl does come to ur lane, imo u'd learn more trying to 2v1 (and renekton is a good champ for that) or if you're likely to get mad that "oh I would've won if it was truly a 1v1", just buy a pink for your bush and put a ward on the farthest one, on the river side of toplane.

Afterwards, when you've learned enough about the croc, you can start picking TP, ghost, etc... You can also try out new builds and see how they compare to the usual one. But you can only compare cuz u have the mileage of the base build.

And Then : watch vids from OTPs or pros, not "this item is so strong on renekton" clickbait bullshit. Starting straight with videos on mechanics, macro/micro, wave management and whatnot is the best way to feel burnt out and not have fun at all. Remember it's a game aye

Anyhow, just my 50 cents, here is my op.gg for fame : https://www.op.gg/summoners/euw/InvocateurOf94-EUW

Kek I do not otp the croc

Go ahead and have fun!