Sim locked

How do I find what carrier an iPhone is locked to

Ask that 'someone'

See this Apple article: Use a SIM PIN for your iPhone or iPad

Use a SIM PIN for your iPhone or iPad - Apple Support

OR

See this Apple article: How to unlock your iPhone for use with a different carrier

How to unlock your iPhone for use with a different carrier - Apple Support

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.
• You need to contact your Carrier.

I just bought it from someone

The best way to get that iPhone ready for you is contact that 'someone' (= previous owner ?)

The previous owner has to read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Then the previous owner has to remove the iPhone from his account, see this Apple article:

Check your Apple ID device list to see where you're signed in

Check your Apple ID device list to see where you're signed in - Apple Support

Then you can sign in with own your Apple ID.

See the following Apple articles:

Activation Lock for iPhone, iPad, and iPod touch

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

How to remove Activation Lock

How to remove Activation Lock – Apple Support (UK)

• If you need help removing Activation Lock and have proof of purchase documentation, you can start an Activation Lock support request.

You can also contact Apple support for help, -> Get Support (apple.com)

You have to show proof of ownership of your product, provide all documentation showing your purchase of the device.

If you do not have additional proof of ownership, contact the reseller. They may be able to provide the original purchase document.

Once Apple receive your proof of ownership, Apple will further review your request and provide an update. 

Or ask your money back, see: The All Too Common SAD Reality of Buying a Used iPad/iPhone

The All Too Common SAD Reality of Buying … - Apple Community