What is the best thing to build?
Should I build both armor and magic resist?
Should I just focus on one of the two?
Should I build HP heavy items?
Or should I just accept my fate, buy mobility boots, and run it down mid?
Any tank players that could help me with this predicament?
1wolfpack1 votesstart off by building for your lane opponent. but as the game progresses look at who is feed on the other team and build accordingly.
Flaherty1 votesDepends on how well certain champions on their team are doing. Is their ap super fed? get more mr. is there adc super fed? get more armor. If everyone on their team is performing the same then get some of both and build some more hp.
ValyrianBlade1 votesIn addition to the other points - look at who is prioritizing you and who is instead diving past you.
They have 2 AP damage dealers but one is too busy peeling off your assassin while the other is going for your ADC every fight? Then prioritize armor.
Also look at who you can knock out fast in a fight. They have two good AD champions but one is diving your back-line and you can CC the other until he's dead? Prioritize MR and health (with maybe a little armor).
In general, I like to get 2 items for the stat I'm more concerned about, 1 item for the less concerned stat, and then health / damage otherwise (I'm speaking bruisers here to be fair).
E.g. core of 
or 
Hrd2H1t1 votesThis is actually super helpful, thank you