To unlock a Link Summon, you need to obtain a Heartbinder item, and once you obtain that, you are able to summon the character to fight alongside you later.
You can use a Link Summon in battle only if you have a 100 % full MP bar, so stock up on some Ethers if you need to quickly fill it. You won't have to worry about your health bar tough, since this will be refilled when you use a Link Summon, so it's great to use it when you are already at low health.
All the different Link Summons has completely different abilities, so you will naturally end up liking some of them more than others. The best way is simply to try them all a few times, and stick to the ones you like the best. Some are also better at certain situations than others, so use this to your advantage during difficult encounters.
How to unlock all five Link SummonsThere are a total of 5 different Link Summons in Kingdom Hearts 3, and you will get them all by playing the main story. Some of them won't be unlocked until many hours into the game tough.
Anyway, here are all the unlockable Link Summons in Kingdom Hearts 3. There are some spoilers below, so don't read this if you want to know nothing about the story.