in case of alcohols:
OH is a BAD BAD BAD leaving group, there are two ways to make it a good leaving group:
A) through SN1: protonate it then becomes water and water is a good leaving group, whay SN1, because you probabely have a 3 carbon
B) through SN2: convert the alcohol to tosylate using tosyl chloride,now you have an excellent leaving group, why SN2: always look at the carbon attached to the leaving group....in this case you most likely have a secondary carbon
hpe this helps
good luck to you!