To be an author on a scientific paper, you are supposed to have done 3 things; 1) been involved with creating the data, 2) been involved with interpreting the data and 3) been involved with writing the paper. Hence, it is not unreasonable to expect input from your supervisor (and others who are expected to co-author). If you are doing the bulk of the work, then it is reasonable that you are first author (with the descending list of authors reflecting their level of input, but head of group will usually go last). The above is a general guideline, best to chat to your supervisor about any worries. Good luck, paper writing is hard, especially at the beginning but this is where you want to take advantage of the experience of your supervisors who should be able (and hopefully very willing) to guide you through it.