Try the below sample code:

PAGE:

<apex:page controller="controller_picklistDemo">
<apex:form >
    <apex:pageBlock mode="edit">
        <apex:pageBlockButtons location="top">
            <apex:selectList multiselect="false" size="1" value="{!selectedValue}">
                <apex:selectOptions value="{!options}"/>
                <apex:actionSupport event="onchange" reRender="pbSection" status="pStatus"/>
            </apex:selectList>
            <apex:actionStatus id="pStatus" startText="Rendering..."/>
        </apex:pageBlockButtons>
        <apex:pageBlockSection id="pbSection" columns="1">
            <apex:pageBlockSectionItem rendered="{!selectedValue='Credit'}">
                <apex:outputLabel value="Credit:"/>
                <apex:inputText />
            </apex:pageBlockSectionItem>
            <apex:pageBlockSectionItem rendered="{!selectedValue='Debit'}">
                <apex:outputLabel value="Debit:"/>
                <apex:inputText />
            </apex:pageBlockSectionItem>
        </apex:pageBlockSection>
    </apex:pageBlock>
</apex:form>
</apex:page>

CONTROLLER:

public class controller_picklistDemo {
    //Picklist selected Value
    public String selectedValue { get; set; }
    
    //Picklist values
    public List<SelectOption> getOptions() {
        List<SelectOption> options = new List<SelectOption>();
        options.add(new SelectOption('Credit','Credit'));
        options.add(new SelectOption('Debit','Debit'));
        return options;
    }
    
    //Constructor to initialize picklist value
    public controller_picklistDemo() {
        selectedValue = 'Credit';
    }
}