Graphic designers moving into web design has become a very common path. What you really need to prepare is not software operation, but two things: understanding that pages change, and understanding that pages are for users to interact with.
Core difference: from a single image to a flow
Graphic work is static. After delivery, how the viewer's gaze moves and how long they look is determined by the image itself. A web page is different: it must consider where users come in, what they see, where they click, and where they go next. The same page also appears differently on different screens. In other words, the object of graphic design work is a single image, while the object of web design work is a flow.
This difference brings a series of concrete effects. Typography must account for expansion and contraction based on content amount; color must account for interaction states (default, hover, click, disabled); visuals must account for information that only appears after scrolling. Many people transitioning from graphic design get stuck at this step, not because they can't do the work, but because they habitually treat the page as an enlarged poster.
Skills to fill in
The first is responsive thinking, meaning one design must hold up across different widths. The second is basic page knowledge: at least knowing the respective responsibilities of structure, styles, and scripts, which determines whether you can communicate smoothly with developers. The third is the basic concept of content management: understanding how content is entered, organized, and retrieved. Many design problems can be avoided at the source.
These don't need to be learned to the level of being able to develop, but they do need to be learned to the level of being able to have a conversation. Think of your ability structure as a "T": the vertical stroke is still the brand visuals and aesthetic judgment you were originally good at, while the horizontal stroke adds layout, interaction, and a bit of engineering concepts. The deeper the vertical, the more valuable the horizontal additions are.
A few common pitfalls
The most common is letting visual effects overpower usability. Full-screen motion graphics and complex scroll effects are impressive in a showcase, but once they affect loading speed or interfere with accessing information, they become a burden. Next is unclear interaction logic: skipping levels in the entry hierarchy, hiding important actions in corners—users will leave within a few steps.
Another is designing only a single screen. Every screen looks good in the design file, but when put together they lack coherence; during scrolling the focus keeps changing, and users can't remember what the page is saying.
A pragmatic path
A workable approach is to first practice with an existing project: adapt a graphic design into a page that can fit three widths, then try designing two or three interaction states for it. In the process, focus on experiencing not "looks better" but "works better." Once familiar, you can start with showcase-type pages for real needs; this type of page has a relatively simple structure and a large margin for error, making it suitable for accumulating experience.
The essence of transitioning is not abandoning your past abilities, but giving your aesthetic judgment one more place to land. Designers who can understand both brand expression and user interaction will be far more important in projects than those who merely provide visual comps.