You also have the ability to put some validation onto your controls using some of the available validation controls.
RequiredFieldValidator will check if the control you assigned to it (ControlToValidate) has a value. Other properties you can set are Text, and ErrorMessage. Text is displayed when no value is entered for the control while ErrorMessage will show up on the ValidationSummary control if you have it setup on your webpage.
CompareValidator will compare the value of the control you assigned to it (ControlToValidate) to some value you specify (ValueToCompare) given the comparison operator you indicated (Operator). You can also compare it to another control (ControlToCompare). You also need to set the Type to indicate the type of the values that you want to compare (e.g. String, Integer, Double, Date, or Currency). As with the RequiredFieldValidator, you can also set the Text and ErrorMessage properties.
If you want to align your controls or lay them out in relation to the other controls, you can make use of the Layout toolbar. Go to View -> Toolbars and select Layout. You can align lefts, centers, rights, tops, middles, and bottoms, make same width, height, and size, increase/decrease/remove horizontal/vertical spacing, and bring to front/send to back.
If you need to postback to the server after selection to the control is changed without having the user to initiate the postback (using a Button control), you can set the AutoPostBack property to True.