To be a good User Experience Designer, you have to understand the customer/ user and the problem. The key is to solve the problem by the help of design. You have to understand the various aspects of User Experience design and find a niche and complement it with other skills. Broadly UX can be categorized into
User Research - This is one of the key part of UX, User research is done to understand your customer. Without user research, you may design the best looking website that works great but if no one wants it, then it is going to be a fail. User research can be done in many ways from observation,personal interviews, to user forms and surveys and also user personas. Each designer chooses to do the research in their own way based on the amount of data and access they have.
Content Strategy - One of the most important part of UX, it involves analyzing content, understanding what content users prefer in the given context and how much of it should be text or images etc. The key here is that it has to be useful and usable and consistent.
Information Architecture - Having useful and usable content is not enough if it is not presented in a meaningful way. IA deals with identifying organizing the content in a systematic way so that users can access it with ease and in a way that comes naturally to them.
Interaction Design - ID involves designing interactive behaviours, think actions and reactions and incorporate typical user behaviour. Specific focus must be given to use. Think, if you were in the real world, you could show someone that what they were doing are right or wrong but it's obscure online and hence ID is very important
Visual Design - A lot more than 'just making the website look pretty/cool' you have to understand the psychology of colours and visual appeal,the differences in terms of typography and overall purpose to build an aesthetically pleasing site to your end customers, an example is, you use significantly different colours and fonts for a education portal than for a halloween themed website. Considering aspects of occasion and purpose of use.
Usability and Analytics - Measuring the quality of user experience. Focusing on users and tasks and using metrics to understand what works and what doesn't to change by iterating.
You can either be a generalist and gather significant knowledge in each and every area or you can deep dive into one area at a granular level. You can also pick a few of the above based on your skill levels in each and interest and gain knowledge in other areas to complement your stronger areas.
Lean UX is basically understanding and utilizing agile methodologies to your ux work so that it provides better user experience, to gain more knowledge in the area you can read some books on lean ux such as Lean UX by Jeff Golthef, Agile Experience Design by Lindsay Ratcliffe and Marc Mcneill.
Always keep learning!