1) If you have any tolerance to other opiates like Vicodin it effect your dosage. But assuming you have zero tolerance, I would take 2 of the 5 mg pills (10 mg) to start and see how you feel in about an hour. You might need 3-4 (15-20 mg) because different people react differently, but start off slow. You can always take more later. I'd be willing to bet that at your height, weight and experience, 2 or at the most 3 will have you feeling great.
2) You're going to get a lot of opinions on the best way to take Oxycodone, but for a first timer I'd say just take them orally. A lot of people like to snort oxy, but this can be a) wasteful because less of the drug actually gets absorbed and b) harmful because a lot of the oxycodone 5 mg pills are cut with tylenol. So, to answer your question, I'd say taken Oxy orally is the best way.
3) You can expect some "warm and fuzzy" feeling euphoria, some sedation (although many people feel oxycodone gets them "up"), and some mild cognitive impairment. Ohh yeah, and you might get very nauseous and vomit.
4) Weed will not "fuck the high up," if anything it potentiates it. I'd wait to see how you're feeling with the oxy but I don't think it'll do any harm. IME, weed and opiates go together amazingly well.
Have fun and be safe.
oxy, am pretty jealous, although I'd need a bit more than that these days - tolerance definitely sneaks up on you!