I'm not a Xi'an expert, and the advice @Bryanbai gave you above looks excellent.
But if for some reason that doesn't work out, all you need to do is ask at the front desk of your hotel. It's a frequently asked question, especially by Chinese tourists.
If the hotel has a massage service, it's usually not the best choice. Better to go outside for several reasons. I can expand on that more if you care. Just ask them directions and they can point you to someplace nearby.
The "locals" places will cost under 50 Yuan for a basic one and the places that are more accustomed to tourists should still be between 50 and 100 Yuan. You need a place accustomed to tourists if you don't speak Chinese. They will most likely have an English menu.
Sometimes a foot massage will include attention to your neck, arms, and shoulders. Sometimes even your back. These make it cost a little more, total usually being closer to the 100 mark.
You should always be offered a hot water foot soak first. That's an important part of the process. No extra charge for that, unless you accept some sort of herbal additive to the water. 中药 (Chinese medicine) is usually the cheapest if it's not included free, and others cost more. Ginger 姜 is energizing, and I usually get that.
Sometimes they will offer to use a higher grade flower essence oil 精油 instead of the standard massage lotion compound 按摩膏。 That costs more, maybe an extra 30 Yuan.
I get a foot massage every week at home in Kunming and always seek them out when I travel to other cities in China. Highly recommended for health maintenance as well as for tired feet and general relaxation.