I have to build a transformer which are the main formulas or some source that explains this
You need to specify what the transformer is to be used for. (why do you have to build a transformer ?) (the characteristics of the core are critical to the design) Until you specify that, only the simplest of basics relating to turns ratios can be stated: 1 - Power in primary Power in secondary 2 - Voltage ratio turns ratio 3 - Current ratio 1 / (turns ratio) [from #1 #2] 4 - Impedance ratio Turns ratio squared. [ from #2 #3] **IF** it is a power transformer, I suggest getting a transformer with the same, or slightly higher POWER rating, then simply re-wind the secondary using #2 above. (I did) or Google: how to design transformer windings then pick the one that suits your needs best.
