You need to decide what pool you want to join.
The first one in the above list requires you to register. The last two do not.
Because you did not specify and wish to use in-wallet mining, I will presume you are referring to CPU, not GPU, mining. I will also assume you are running Windows.
You will need to download a CPU miner capable of the x14 algorithm. I suggest: https://github.com/tpruvot/cpuminer-multi/releases/download/v1.1-multi/cpuminer-multi-rel1.1.zip
Extract all the files. You need to find the correct executable version for your CPU. If you have a 32-bit processor, you will find it at cpuminer-multi-rel1.1\x86\cpuminer-x86.exe in those files you just extracted. If you have a 64-bit processor, you will find it at cpuminer-multi-rel1.1\cpuminer-x64.exe in those files you just extracted. If you are uncertain, use the 32-bit one. Even if you have a 64-bit processor, it will run the 32-bit version, just slightly less sufficiently.
Now that you know which executable to use, rename it to "minerd.exe" and copy it to the same directory as the BERNcash-qt.exe
Now you are ready for the wallet to issue commands to the minerd.exe you just moved. Open your BERN wallet, click "Mining", select "pool mining", use 4 threads, use a scantime of 10, and fill in the Server for the pool you selected (i.e. stratum+tcp://aikapool.com), port the pool uses (i.e. 7947), your worker name, and your worker password.
All of the above mentioned pools have documentation that explains the Server, Port, Username, Password to use. It is a bit tricky to setup at first, be patient and remain calm. You will get it. If you run into a question, feel free to ask. Myself or another community member will respond ASAP!