Make sure you have easy roam enabled ahead of time. You can also text ROAM to 7626 to enable it.

https://www.koodomobile.com/easyroam

Once that's done, in the US, it'll be 9 dollars a 24 hour period to a maximum of 100 dollars per billing cycle.
Keep that in mind if you'll be traveling between two billing cycles to not be surprised.

Will I automatically connect when I am in the US? Is there anything special that I need to do?

Normally it should automatically connect to a US cellular network. If the phone doesn't, try to restart and reset network settings of the phone. Also, if you are using easyroam, make sure you enable data roaming in the phone. 

Thank you.

@MesaMac Are you on  postpaid plan? Meaning, you have a monthly bill vs buying minutes, data etc. when you need it. If you are a prepaid customer the process to enable Easy Roam is a little different, as you would need a US Roaming booster add-on:

https://www.koodomobile.com/prepaid-plans

If you are on a monthly plan pls ignore what I wrote above. :-)  The other customers above have given you all the right info.

I get a bill every month for unlimited talk and text and 3G of data. I assume that means that I have a ‘postpaid plan’? And that all I need to do is to “Add-on’ Easy Roam to my account for US travel?

Correct.

Once you enter US, you should receive notification too ( if it's not enabled).