Hi AoiMori,
Firstly, if you're a beginner at HPLC have you been trained in using the instrument at all with someone at your facility? It is much handier to learn from an expert that is directly teaching you than someone on the forum (if that is even possible). If you do, I would recommend immediately asking them for assistance as they will give you better answers, are likely more familiar with your task/application, and hopefully have years of experience with LCs.
As for the question itself, you will need to provide more information. I would suggest uploading a picture as recommended in the stickied post at the top of the forum. However this is just the start, information on your system, gradient, sample prep, etc. will be helpful in getting better advice from users on the forum.
If internal/external standards, calibration curves, etc are used include them too.
From what I read: low peaks at the beginning of the method could be a few things, normal disruption from the act of injection itself, mobile-phase sample-diluent differences will also cause a peak, as well as any unretained material that shoots straight through the column with no retention. However, without knowing much more, we cannot be much of assistance.
Please provide us with some more information and hopefully you will get a more detailed response
