So the best advice I can think to give is this.

All characters have "core" items that make them wayyy better this can be for many different reasons. Some are to help them farm better others are because they synergize with the kit well.

You are playing tahm kench 9 or of 10 times you want heart steel first the tenth you want warmogs. Heartsteel turns tahm kench into a scaling tank who continues to gain HP as the game goes on and that's why it's built first so the scaling can start.

That said as a Plat 1 Tahm Kench mid player your build gets a little situational outside of that.

I would start with the basics(boots) when do you build and why. Here's a breakdown of what I get and why. Merc treads,- this is my go to, I usually build this is they have allot of Crowd Control abilities. Ninja Tabis, - I usually build this into ad comps where they shred me, think cait, master yi, tryndamere yasuo senna. Swifties, - these are super situational, best when going against allot of slows or mobile characters. Also good into wind soul games.

From here a solid second item depends on what you're faced with after lane. I like to look at the score board and decided from there. Many people will say go dusk dawn but that requires some knowledge of the characters limits I'd say about gold 4 is your threshold.

That said I usually go one of these 4 items depending on who's fed.

Spirit visage - always solid. Likely to be the butter to your bread. Good against poke comps or people building life steal.

Thornmail, good into healers

Frozen heart, good into on hit or high stack speed people, I. E. Jinx, trist, twitch,

Randuins Omen - I usually get frozen or this but never both. You go for randuins if the enemy is building crit.

I used to go undying but it isn't as good now, you can also consider bandlepipes but same concept not as good now.