I would start by revising the alphabet and checking whether she knows numbers. You can then teach and practise the verb to be. You can do this by teaching ‘my name is.....’, then progress to nationalities and later occupations. Teaching nationalities and jobs will also build her vocabulary. Remember to take it slowly at this level and include a lot of revision. You can teach the affirmative of the verb ‘to be’ in one lesson; move onto questions and revision of affirmative in the next lesson and then work on negatives. It’s a good idea to use a course book at this level; it’s probably more important than at any other level, especially if you are a new teacher. The book will provide a syllabus and structure for what you’re going to teach.
As for which language you should use, I suggest you use English. As you say, it’s important that she hears as much English as possible. At beginner level, this is possible through the use of mime and pictures when teaching vocabulary and short phrases. When teaching grammar, teach as much as possible by showing examples. If your student doesn’t understand all the examples, you can periodically use Russian. A lot of time can be saved through the use of your student’s mother tongue but this should be treated as a last resort.