Your plants can only develop fully if they are supplied with sufficient nutrients. For this reason, fertilizer is a must!
If you are a beginner and have no experience with grow fertilizers, we recommend a Hesi starter set. With it you will achieve very good results in any case. The Hesi Starter Set comes with an illustrated manual (Hesi Grow Scheme). It tells you exactly how much of which fertilizer to use and when.
If you do not want to use mineral fertilizers, but want to fertilize purely on a biological basis, then we recommend a BioBizz Starter Pack. Here you also get an illustrated instruction. You can't do anything wrong with it. :)