You are a new teacher and it is normal to be feeling afraid. The fact that you are concerned about your pupils shows that you care which is a very important factor in teaching. Also remember that you were given this opportunity for a reason - the person who made the offer believes you can succeed.
As you are concerned about your level of English, I’m guessing you are not a native speaker. You can start working on your own language by reading in English. By this I mean reading for pleasure. Just read anything that you enjoy and you will absorb more of the language, you’ll learn things without having to think too hard about it. When you come across a something that is new to you, you can study the construction of the phrase; also pay attention to spelling, punctuation, etc. You can also watch films, listen to the radio, etc. If you immerse yourself in English for a short period every day (or almost every day), you will make progress.
As for the girl from the USA, I don’t think she will expect you to have the level of a native English speaker. Spend some time speaking to her to get to know her as a person. Explain to her that you are a new teacher and that you want everything to go well. Tell her that you are willing to learn. You can tell her how you are feeling: maybe nervous, stressed or uncomfortable. Sometimes just giving voice to your feelings can help and she will probably be supportive.
To motivate your students, you should have plenty of variety in the classroom. Look for activities and topics that interest teenagers – you can look at some of my replies on this subject. Teens need to see a balance between serious work and games or fun activities. They like to be treated seriously as adults and also need opportunities for fun.
As the students are fourth year of high school, I guess they have a coursebook. To check the students’ level, you can have them do exercises from the beginning of this year’s coursebook and the end of last year’s coursebook (which you should be able to get hold of). You can spend some time consolidating the work done last year but you will need to move onto the syllabus for this year pretty quickly because the students will probably be graded on this at the end of the school year.
I hope this will help you in your new job. Please write in again, if you would like more ideas.