If you started exploring Concurrency recently or even trading from long time however now want to get in to mining, you most likely would have come across this question. If you do a search on Concurrency forums, you would typically get answer like" This is million dollar question" or go to whattomine to get your answers. To someone putting their toes in the water and do not have lot of time to research, I am afraid there are not a lot of answers out.
While I don't have an one liner answer to this million dollar question, after stumbling myself for few months I have come up with some answers that may help steer you towards right direction.
Disclaimer : I do not claim to know all and this is something I have learn and using. Please do your own DD before investing any money of Mining hardware or any thing else!!
There are plenty of help out there to help you build a mining rig. Please remember you either need a ASIC based hardware or a mining rig with minimum 2-3 GPUs to see any meaningful return. I would recommend going with high end card (such as 1080 Ti) or ASIC with highest hash rates. Unless you have free electricity, old ASIC or low end GPU are not going to help.
Now the main question, what to mine. While most of the ASICs are build for specific alog, this is not a big question. You can check for profiltability of those at https://www.cryptocompare.com/mining/calculator or check for profitability at Nicehash to see where you want to point your hardware to. The instructions for those are easily available online.
If you are more adventures like me and either already have or decided to build a mining rig, this is a big deal as what to mine to get most profile. Easy way out is to pick and existing popular coin such as ETH, Monero, Zcash etc and make decent profit.
What I found best is to look for a new coin and not only make better then decent profit mining today, but also find something that may give me bigger gain in future (if we hold the coin for some time). One of the way I have found this to work for me is look at Coinmarketcap.com.
Go to coinmarketcap.com go to trading - recently added. You would come across a list of coins added in last 30 days. Look for ones that do not have * next to then as they are the only ones you can mine. Go to the website of that coin to see if minable does means you can do PoW as PoS is also called minable however you may not able to mine those coins using your rig.
You have to study the website as well as comments/reviews etc on forum like Bitcointalk or Steemit to see if this new coin looks promising. I am afraid this is something no one can teach or answer. Post you narrow down to 2-3 coins, you want to try to understand if mining this coin will give you right profitability. For that, look at Algo this coin it using with help from what to mine, you should be able to figure out hash rate you would get with your rig and profitability of this new coin using your rig. If this info is not available, I personally try to mine it for 6- 8 hours to see if I am wasting my time or there is some meat in it. I am hoping you have some info from your couple of days of mining known coins to see how much money your rig makes per day and how much you will make using new coin.
Another way that you could look for new Algo or coins is to go to big mining pools (most don't need you to register to see some basic pool info and registration generally does not take more then couple of mins). What you are looking is pool status where it shows status for all the coin and look which coin is mined by most miner. Eliminate the one that needs ASIC or have a ASIC based hardware out as that is not very profitable for your GPU miner.
Hope this helps you decide what to mine and make you some Greens!! Happy mining !!!