Have deep rooted negative beliefs about themselves/identity: Such as 'I'm not good enough' and 'I'm unlovable'. These types of beliefs are taken as fact rather than being recognised as opinions they have of themselves.
Frequently criticise themselves: They may put themselves down frequently, or joke about themselves in a negative way. They may also constantly blame themselves when things go wrong or doubt themselves
Ignore their positive qualities: People with low self esteem may not accept compliments, instead, attributing their achievements to good luck or downplaying them/brushing them aside. Additionally, they may focus on what they didn't do or haven't achieved, and mistakes they may have made.
They may also experience emotions such as anger, frustration, sadness, anxiety and may feel depressed, guilty or ashamed

Their feelings and beliefs about themselves may impact on their relationships and work, their ability to engage in activities and their personal care. They may isolate themselves, become upset or distressed by criticism, or always try to please others. They may be unable to stand up for themselves in their interactions with others; conversely they may be overly aggressive in their interactions with others.

Low self esteem can be maintained by a number of factors

Negative thinking: People with low self esteem may adopt unhelpful thinking styles which maintain their low sense of self worth. They may focus on the negative aspects of a situation and ignore the positives. They may also anticipate the worst possible outcome, or place blame on themselves when things go wrong.

Unhelpful behaviours: Certain behaviours we adopt may worsen or maintain low self esteem. These may involve avoidance of certain situations due to negative beliefs one might hold.

How do I begin to address this?

Low self esteem is commonly experienced by many individuals, and the good news is, we develop healthier self-esteem through many different avenues. As a first step, it might be helpful to talk with someone to make sense of your individual experience and think together about ways to address this. There are also some tips one can adopt to begin the journey to healthier self esteem:

Start with a list of positive qualities: Whilst this may initially be difficult to do, try and list all your positive qualities, no matter how small or insignificant they may seem. You can ask yourself questions such like 'What positive characteristics do I have? 'What are some skills or talents I have?' 'If someone shared my identical characteristics, what would I admire in them?'
Maintain a positive 'you' journal: In a positive 'you' journal, note down daily examples where you have demonstrated the positive qualities you have listed above. Start to notice your qualities on a daily basis
Challenge unhelpful thinking: You can do this by utilising thought diaries, question unhelpful thinking, and try to develop an alternative more balanced approach. Ask yourself 'Am I being fair to myself in thinking in this way?' 'What might be a different way I can think about this?'
Adjust Beliefs: Try to develop more balanced beliefs and behave in ways which support these new beliefs. For instance, developing a belief of 'I am good enough' might correspond with more compassionate behaviour such as asserting yourself in interactions with others.
Engage in enjoyable activities: Try new things, try to engage in hobbies and interests which allow you to explore your identity and elevate your mood
Take care of yourself: Try to do nice things for yourself, be considerate and compassionate to yourself.
Test your fears: Avoid 'avoidance' and isolation, approach situations with an open mind. Try and engage with life, live in the moment, and take note of the small steps you make.

Further details

Books

Overcoming low self esteem - a self-help guide using cognitive behavioural techniques: Melanie Fennell
Self-esteem bible: build your confidence day by day by Gael Lindenfield