Can't import even basic things like minecraft.entity

My friend said that "you need to rebuild alot of the mod then since most of it is depreciated". But I don't know how to do this. i

I imported this and now I have errors:package net.minecraft.entity does not exist; package net.minecraft.item does not exist

import net.minecraft.entity.Entity;
import net.minecraft.entity.LivingEntity;
import net.minecraft.item.ItemStack;