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https://www.dndbeyond.com/sources/basic-rules

There’s a link to the basic rules. At the most basic example, let’s say you swing your sword at a goblin. The goblin has an Armor Class, or AC, of 15, thanks to his leather armor and shield. So in order for your swing to hit, you have to get higher than 15. So you roll a d20, and it lands on 13. However, since you’re good at swinging swords and very strong, you can add those bonuses to your roll. Let’s say +3. So your roll ends up being a 16, which beats 15. So then you roll a couple dice to see how much damage you do based on the type of weapon and how good you swing swords again. For a long sword you would roll a d8 and add any relevant modifiers to your roll. Then you subtract that number from the enemy’s go. When it’s the goblin’s turn, the DM rolls against your AC to do damage to you.

Those are the basics to all of the rolling. Someone rolls a d20 and adds modifiers to determine if they are successful at something against a target number, and the other dice are mostly for damage, healing, or to choose something random on a table.

It sounds complicated as a comment but when you play you have your character sheet which shows what all the numbers are supposed to be.

Edit: for your given example, there isn’t an ice shield spell, but there is a spell called Armor of Agathys that covers your character in frost. It gives you 5 temporary hp, and if a creature hits you in melee range (they roll higher than your AC) they take 5 damage.

Each spell has a specific description of its effects. You can cast spells at different levels depending on how strong they are, and it costs spell slots (resource points) to cast them.