"I am wanting one boat."
Translation:Tha mi ag iarraidh aon bhàta.
Okay, first it's 'bàta' not 'bata' (which would be a different word). As for the lenition, you use it whenever the grammar causes lenition. In this case, 'aon' always lenites the noun it is qualifying. ('bàta', a boat; 'aon bhàta', one boat)
Lenition itself is: any consonant except L,N,R adds an 'h' and changes sound.