My iPhone 4 has no sim card slot, how do I put a sim in?

I got it off eBay from America, and there's no sim card tray, how do I out my sim in it?

It's built in.

It is built in. I'm not sure if there's an actual slot, or its programmed into the phone. Anyways, carefully try to take it apart, if you can't do that you should sell it and get some of your money back.

Here's your solution if its the sim card from a previous phone:
If you need to get your old sim card in it, to get the data transferred over take it into the carrier or Apple store and they can switch the sim card for you (if its possible, it depends on the condition and type of sim card). You could try doing it yourself but there's a chance, if you don't know how to do it and/or have never done it before, that something may get messed up. You could also bring in your old phone to the carrier store and have them transfer all the data, or you could use your computer depending on what kind of phone you had previously and if you still have it with all the data on it.

That means you got a CDMA phone, not the GSM model which has a SIM card slot. You can't put a SIM into it since it's a different type of phone.

Well, you may have an iPhone for Verizon or Sprint, which doesn't use a SIM card. Those phones are for CDMA networks, which don't use SIM cards.

If it doesn't have a SIM slot, you can't put any SIM in it. You should've checked before buying it. You can just use it as iPod.