Your class hands you a starting kit, and offers you a pile of gold instead. Your background adds a smaller set of its own. That is the whole of this step, and there is really only one decision in it.
Each class offers two or three starting packages — a specific set of armour, weapons, tools and a pack, each with a little gold on top — or one option that is nothing but gold, usually around 150 GP.
Take the kit. It is assembled to make a 1st-level character of that class work on the first evening: the armour is armour you are trained in, the weapon suits your primary ability, and the pack contains the unglamorous things that turn out to matter. Buying the same contents piecemeal costs more than the gold option gives you, and it costs an hour.
The gold is there for players who know exactly what they want and why — usually a specific weapon the kit does not include. That is a real reason. "I will work it out later" is not, and it is how characters arrive at the table without rope.
Armour is the one place where taking something you like the look of has teeth. Your armor training comes from your class, and wearing armour you lack training in is not a small penalty:
The same principle applies more gently to a weapon you are untrained with: you can swing it, you simply add no proficiency bonus to the attack. Nothing stops you carrying either. The rules just decline to help.
Armour Class is what an attacker has to beat. Heavier armour gives a higher base number and cares less about your Dexterity; lighter armour gives a lower one and adds your Dexterity modifier on top. Which is better depends entirely on the character you built two steps ago — a high-Dexterity character in light armour can match a heavily armoured one, and moves faster doing it.
Your kit already resolves this correctly for your class. It is worth understanding anyway, because the first magic armour you find will ask you the question again.