Measure your pH and kH then calculate your CO2 level using the graph
here.
A good goal CO2 to aim for is about 30.
I suggest DIY CO2 for this size of tank.
What worked well for me is run DIY CO2 for 3 months or so, when the plants have grown in well discontinue the CO2 and you will have a nice planted tank that doesn't need too much pruning.
The amount of bubbles per minute depends on how you are diffusing your CO2 as well as surface agitation in the tank. For DIY CO2 you have no control over bubbles per minute. For CO2 injection with regulator and needle valve for that size of tank you could start with one bubble every 5 seconds, then adjust the rate up gradually depending on your CO2 level, allow it several hours to a day between adjustments. On my 75g 1 bubble per second works well.