Tower Controller in training reported an NMAC between two aircraft in the traffic pattern.

Narrative

Both planes were staying in the pattern. Aircraft Y requested Runway X and Aircraft X requested Y. Aircraft Y departed first and I put him in left traffic to avoid the departure of the Aircraft X; when Aircraft X departed I put him in right traffic at the time initially thinking if I put him in left traffic that he would quickly catch up to Aircraft Y and become an issue. I called traffic to the Aircraft X about Aircraft Y and he did not have him in sight and asked the order to the field when I told him he would be number 1 to the field with both aircraft about midfield downwind at this point and Aircraft X; 500 feet above Aircraft Y. I then called traffic to Aircraft Y about Aircraft X and he had him in sight. I gave Aircraft X clearance for the option at this point on Runway Y and then to give a little more spacing I told Aircraft Y to extend his downwind and that I would call his base for Runway X. As Aircraft X was descending and turning base he flew towards Aircraft Y who was extending downwind and Aircraft Y maneuvered to the left to avoid him and Aircraft X told me he was going around. After the incident I talked to a fellow CPC and a Supervisor and found a few things I could have done better to have avoided that situation and to have dealt with the conflict as it was arising.For starters putting Aircraft X in left traffic would have been the better option rather than right which caused the planes to be going nose to nose towards each other. Also when Aircraft Y had traffic in sight I should have told him to maintain visual separation. I also should have changed the sequence and extended Aircraft X downwind while I let Aircraft Y make a short approach and come in #1 to keep Aircraft X on the outside of Aircraft Y's flight path. I also learned a little more about the characteristics of Aircraft X such as its speed in the pattern and how it descends and makes a much more rounded turn from downwind to final rather than squared turns to base and then again to final.