Drop down basics

I suspect that what you'd like to do is populate a Drop Down with items found on another list. You can easily do this using a List Lookup Control which is found in the SharePoint section of the Form Controls panel:

Once you have dragged it onto your Form, you'll then need to update its settings to something valid before you can publish your form, as it is not configured by default (note: you can still SAVE your form even with these controls in their default unconfigured state!):

Example

Let's say I have a list called Fruits List, and I wanna get a drop down with all of the Titles:

First you can configure your List Lookup Control to target the Site of the List in question by clicking on the green arrow, and then selecting the Site from the list of available sites:

(after clicking the green arrow, a navigation window will pop up that shows your entire site collection. Click on the target site to highlight it then click on the OK button)

Afterwards select your target List in the Source List drop down:

Lastly set the List Column Name to whichever Column that you'd like to target. In my case I want to use the default Title column as the stuff I'll be putting into my dropdown:

Save the Settings and preview your form to see how it looks!:

I hope that this helps you accomplish what it is you were trying to do! Let me know if you need any further assistance.