Adobe JavaScript Form Question - New to JS

[STLC Programmer]

Brand new to JS.

Working in Adobe, attempting to take a drop down field and based on the value selected I want to use a 3 digit # into a text field that is beside it. (it will be read only)

I do not have a clue how to start this and I am not sure how to search for help on it since I am not clear what I am asking for?

Anyone willing to point me in the right direction or explain how this script should look?

Thank you in advance.

[WolfShade]

Something like:

<script type="text/javascript">
  function updateRO(a) {
    document.forms["thisForm"].displayNumber.value = a;
    }
</script>
<form name="thisForm" id="thisForm">
  <select name="thisSelect" id="thisSelect" onChange="updateRO(this.value);">
    <option value="">Select</option>
    <option value="123">123</option>
    <option value="456">456</option>
    <option value="789">789</option
  </select>
  <input type="text" readonly="true" name="displayNumber" id="displayNumber" size=3 />
</form>

[STLC Programmer]

I may not have been clear enough in my initial request for assistance.

I am using Adobe Acrobat Pro X. I created an electronic version of our current paper time sheet.

Within Adobe fields, under properties, there is a Validate and Calculate tab.

My goal: Based on a previous field (drop down), dependent upon the user's selection the adjacent field will populate a corresponding # to the previous field automatically.

I do not know how to do this and I do not know enough Java or how to write it to make this work.

All Adobe help files and training videos do not go into this area past showing the tabs so I am not sure how to figure this out.

My current example:

Column1 Column2

Regular Pay 100

I want 100 to populate if Regular Pay is selected.

Any ideas? Links?