Answer
if there is grout on them and the grout is hard already you are going to have a hard time because ive tried to get that stuff off whit a lot of water and a lot of patience but once the grout is hard you can't get it off now if there is no grout on the tiles just carefully ( without dropping them and breaking them) clean the dirt off whit mild soap depending on how dirty they are , maybe do a test tile ) and water and a not too abrasive brush or just bare hands (if you can stand it) in case there is a relief pattern on them make sure you install them levelled because they will crack whit weight afterwards and make sure you clean the grout line & fill them properly & treat the grout lines whit the stuff that protects them ( sorry i don't have the proper term for that one !) i believe home depot , home hardware or something offers detailed instruction (possibly a class and free ) so that you can learn the proper step on how to do that now believe me its is a lot cheaper to install tile by yourself and it is a lot cheaper in the long run if it is done correctly good luck