Hi @lildiy,
You'll have to identify the source of the leak on the inside of the shower. You can seal gaps on the outside, but water will still get in underneath the frame and, over time, cause mould and corrosion.
Once you have identified the source of the leak, apply a bead of Selleys 300g Clear Wet Area Waterproof Silicone Sealant to cover and seal the gap. If there is already degraded silicone in place, it is best to remove it so you have a clean surface to work with.
Check out How To Silicone a Gap for some guidance on the process.
Let me know if you have any further questions.
Jacob
