HI,
You are able to enforce the predecessor relationship, which will prevent the successor task from being started (i.e., status being changed to "In Progress") until the predecessing task is complete. You can read more on this functionality here.
When entering your predecessor task number, simply put "e" after it. i.e., If task 2 was dependant on task 1, I would type "1e" in the predecessor column.
Best Regards,
Rich.