I found some time to put together some steps for you.
As yakuza699 explained, if you have the private key and you are already familiar with bitaddress.org then you can use bitaddress.org to get the info you need.
As an example, I've generated the uncompressed private key:
5KE8nNAVhYKhnNGUYKzK6Z7J2pvzP2jvF6BY272c2HhL8YjCNXbThe uncompressed address for this key is:
1DbU3tK8YB5bNEbsrtWwA86hQEZpaZ4qhgThe compressed address for this key is:
1Fo11qMa3Z849x95Eb6T4qWSeavYrGncvQ
At the bitaddresss.org page, choose "Wallet Details":
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2FepyHbmj.png&t=665&c=geYfWjLM-m47Ug)
On the Wallet Details page, enter the private key that you have and click the "View Details" button:
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2FRndQgh4.png&t=665&c=hD8YJHtI_Y0hnA)
Just below that, you should see the two addresses (compressed and uncompressed) that you have.
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2FaxapdzW.png&t=665&c=fplySU91S3aiNQ)
If you don't see your two addresses, then you have the wrong private key. Either you don't have the private key to the addresses, or you've gotten confused about which key is the right one.
As long as you see your two addresses on the page, then just below that will be BOTH the compressed private key AND the uncompressed private key:
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.imgur.com%2FPnGuDOz.png&t=665&c=4fjBJNtG341IHw)
As you can see, with my example, the compressed private key is:
L3SyMEQufwFZ1EsJZGoxsWinB4ZJGEgS1BsVusc5agh1S4Uq5ReS
Make a copy of whichever key you don't already have. MAKE SURE YOU GET IT EXACTLY RIGHT.
The next step will be to understand what wallet or website you are using to create and push transactions, so we can walk through the necessary steps to use the appropriate private key. Are you building the transactions on bitaddress.org, or are you using a wallet program? If you are using a wallet program, then which one?