If you have two class elements A and B. You can connect them with composition or aggregation relationship. (The difference between composition and aggregation is described here: www.softwareideas.net/a/210/Class-Diagram--UML- )
For one-to-one relationship set the multiplicity as 1 for both roles (ends).
For one-to-many relationship set the multiplicity as 1 for one role (end) and as * for second role (end). You can edit multiplicities directly in the diagram - select a relationship and then click (or double-click) on the top (left or right) blue rectangle over the relationship line. You can type the custom value for multiplicity or you can just choose it from the drop down list.