Toilet Clogs
If your toilet won't flush, there is likely a clog in the system. You can typically dislodge a clog using a plunger. If you don't own one, visit a local hardware or grocery store. You can typically pick one up for a few dollars.
Sink/Shower Clogs
If you sink or shower won't drain properly, there may be hair or other debris in the drain. You can try to clean the drain using a drain cleaning tool; a hand auger can also be used for deeper clogs.
Please don't use drain cleaning chemicals as these may damage the plumbing system.