Question

how ammonia converts into urea

Answer 1

as ammonia is highly toxic base and its accumulation in the body is risky.

but in our body our liver contains some enzymes and carrier molecules which will turn ammonia into urea by using carbon dioxide.
in this process the 2 molecules of ammonia [NH3] is combined with carbon dioxide [CO2]  and creates one molecule of urea. and this process is called UREA CYCLE in which it produces urea ((NH2)2CO).