Hey there, usually xanax or any benzo added to an opiate will sedate you more so than give euphoria. It also will lower breathing and everything else your CNS controls. So be careful/keep an eye out for that or have someone with you. I personally use those types of drugs separately, but if you are dead set on it...usually .5 mg to 1 mg is good for starters on the xanax and 10 mg to 15 of oxy.
Remember you can always dose more, not less. And weed will amplify it, especially since you have zero tolerance. I know personally weed does that for me. Stay safe!