Okay. I'm going to save you a lot of pain and struggle before you delve into the labyrinthine quagmire that is figuring out how Straight Talk works.
Here is what you need to know. Straight talk has contracts with the major carriers. They sell their own Straight Talk phones which you can purchase for full price from their website. They also sell SIM cards designed to work with their service. Those go for around $6.
If you want to bring your own phone to straight talk, you need to get a BYOP sim card for that phone. Each phone is designed to work off of one of the two major cell networks we have in the US: CDMA or GSM. Verizon and I believe Sprint work on CDMA, while AT&T and T-Mobile work on GSM. Maybe there are some phones which can do both, but I am not familiar with those. For the purposes of this explanation, we will assume your phone does one or the other. The way you can tell is.. when people sell their phones, they will say what the original carrier was, e.g., Verizon.
Okay great, so you have a Verizon Galaxy S5 or whatever. Now it MUST be unlocked. Do whatever it is that you have to do to unlock the phone. Maybe you have to take it to the Verizon store to have them unlock it for you (AT&T does this over the phone) or whatever. You must have an "unlocked" phone. This means it can be used by other carriers and not just Verizon.
Once your phone is unlocked -- or if you purchased it that way like I did -- then you get the Straight Talk sim card from the BYOP program. The website will ask which carrier you currently use while it's selling you the sim. You will want to get the Verizon sim card. It will be about $6 and only takes 2-3 days to get to your house.
Now that you have the sim, you power down the phone, insert the sim (and take out the old sim if one exists), then power up the phone. You might want to call straight talk before you do this. There is a phone number and instructions which come with the sim card packaging.
If you want to keep your phone number, DO NOT DISCONNECT the phone from Verizon or other carrier before signing up with straight talk. Assuming you still have service with Verizon, when you call straight talk (on a DIFFERENT PHONE) they will port your number over to their services. Their call center is in like India or Philippines or something, but I've personally never had a problem. The porting process takes a few hours. They say give it up to like 12-24 hours, but it only took about 4-6 hours for me.
I have never had a problem with straight talk, and I'm using a Nexus 5 phone which I purchased used off of Craigslist. It was already unlocked when I purchased it.
You will have to create an account at straight talk's website. Put in your payment information and choose your billing play. I have unlimited everything for $45 per month... had it for the past year, and it is wonderful. I've been a very happy customer. They recently added LTE to their network, so if you are in an appropriate area, you can get LTE service.
One thing to note. Since this is a pre-paid service and not on a contact, if your payment gets declined AT ALL, they will disconnect your service and you will lose your phone number. Make sure to set it up so it bills a credit card or a debit account which will not be declined. I have never had a problem. They automatically send you a text message every time they bill your card for service. I believe you can pre-pay up to like a year in advance, which is very smart because you get a free month of service.
The sim card packaging has all of the instruction, but it can be kind of confusing. You may have a problem initially getting data service. You will need to call straight talk customer service, and they can walk you through the appropriate information to put into the phone so it can connect to the data network.