Can someone explain to me how to do circuit calculations

Use the following rules:

- The sum of currents that enter and exit a node (junction) is always zero. So if you have 3 wires that connect, through one flows 2A, the other 3A, then the third must deliver 5A (taking the direction into account!)

- The sum of voltages across different components should always add up. So if you have a battery of 10V with two unknown resistors, and over one of the resistors is 4V, you know the other one has the remaining 6V.

- With resistors, V=I*R must hold.

With these basic rules you should get a long way!