You will have to unlock the phone to use Net 10 on your cellphone.

Okay I tried the link to AT&T unlock. It says my imei number is invalid. I checked the number on my phone under settings, I also checked by where the battery and Sim card is, also checked the box. All the numbers match to what I typed in. I don't know why it says invalid imei number.
Okay about Net10. Net10 uses AT&T and T-mobile cell towers. They sell phones that can be used on either AT&T or T-mobile. They also sell just Sim cards for either AT&T or T-mobile service.
Net10 is prepaid for $50 a month for unlimited phone, text and 3gb Internet. Can I just buy an AT&T Sim card from Net10 and use that in my LG G2? Or do I have to Unlock it first? If I have to Unlock my phone, how can I do that when AT&T website my imei number is invalid. Thanks for your help.

You are able to unlock your phone after 2 months of postpaid service with AT&T.
att.com/deviceunlock

Then you do not have to worry about using other carrier SIMs, and free to choose Net10, T-Mobile, etc. with the LG G2 device.
Regarding pre-paid SIM, are you wanting a pre-paid device and SIM? or just the SIM?

You have the option to use the LG G2 phone you have and use a pre-paid services from any provider after unlocking.
Endless you plan to return the phone, though AT&T has a 14 day return policy. If you have past that threshold you can either:
1. ETF - Early Termination Fee.
2. Lower your monthly to the lowest of a 2 year contract which is $45 with 300MB.

I would suggest to unlock first, lower your monthly as to the lowest plan, and try not to default.
(as AT&T does block your IMEI number if the event of a default, endless the phone is already unlocked)

I hope this helps.

Okay another question. If I buy a prepaid AT&T Sim card from net10, could I use my phone with that prepaid AT&T Sim card or do I still need to Unlock the phone?

1. GSM carriers can't block phones like CDMA carriers can with ESN numbers. So no, that won't happen. They'll just send your bill to a collection agency and ruin your credit.
2. You would need to unlock the phone (and change the APN settings to use data/mms) to use on net10, yes.

I have a LG g2 on a 2 year contract with AT&T. I can't afford the contract anymore. I have a couple questions.
1. If I default on my contract, will AT&T blacklist my imei number?
2. I want to switch to net10. Net10 uses AT&T or T-mobile for its carriers. Where I live AT&T is the only carrier with good coverage. Do I need to Unlock the phone to put another AT&T Sim card in it? Will the phone work if I use a different AT&T Sim card from net 10?