You should learn the spectrochemical series to know which are weak field ligands and which are strong field ligands. Weak field ligands: (I- , Br- , SCN- , Cl- , F- , OH- , NO2- , H2O). These ligands doesn’t help in the pairing of unpaired electrons. Strong field ligands: (NCS- , NCCH3 , NH3 , NO2- , CN- , NO , CO) These ligands help in pairing of the electrons. Some times there are exceptional case when some weak field ligands acts as strong field ligand for example: [PtCl4]2-, CoF6..Here these ligands act as strong field ligands. Now coming to spin, when there are electrons unpaired in orbital after making the complex which generally happens in case of weak field ligands(there are some exception too) the complex is of high spin which means spin of a complex depends on the number of unpaired electrons in an orbital. On the other hand when all the electrons are paired up generally happens in case of strong field ligands(There are exceptions also) the complex is of low spin. So, spin of a complex is directly proportional to how many electrons are unpaired in orbital.