First,to tap your colony ship,there should be some options you can choose around the bottom corner.Select the "Single turn Action",then there should be some more options you can choose,by then you should be able to find a "colonize" button,however :
If the M class planet you see is red,then it needs to be terraformed
if its green then you already own the said planet
it needs to be grey/white,then you maybe be able to colonize it