I have an old bmx and my chain broke yesterday. I bought a new chain from WalMart. It is 300 links for 1 speed but I don't know what is the bmx speed (gear); it was rusted but I fixed it up. I left my bmx at a program that opens tomorrow but I'm afraid its the wrong chain.
That chain should work OK. 300 links means probably a halflinks chain. Most BMX bikes are 1 speed and many use half link chains.
You will need a chain breaker though to get the length right. To get the length right, count the old links - DO NOT measure the actual length of the old one as the chain could be worn (stretched).