Creatures with "untap" abilities. Combo in mind is "(order of whiteclay)" + Martyr of sands?

I'll make this as short and simple as possible. Card example: Order of Whiteclay: 1/4 creature with the ability of return a creature with 3 of less converted mana cost back from the gy to play. this is an untap ability, meaning it's the og of the tap ability. how do i play this? My main question is, how do i get this creature into the "tap" phase in order to use it's "untap" ability. Can i just call out "i tap this creature" during my turn? Or do i have to actually attack/defend with this creature before it is in "tap mode" for me to "untap" during my turn? If my question is confusing please let me know i'll try and rephrase it in a better way. Thank you all in advance