Clearly, a work that is as long as this novel is will have a number of themes and will contain a number of different symbols. The answer given above refers to a number of the themes, so I will consider an important symbol as an example to help you go back to the novel and identify some others. One of the most notable symbols that appears in this book is that of the sea. Interestingly, the sea in a number of different works of literature is a symbol of mystery or of the unknown, such as in The Awakening. However, in this novel, the sea is a symbol of a powerful force that is connected with death. If we think about it, it is the sea that took Little Em'ly's father in an accident over which she had no control whatsover. Also, it is the sea that is responsible for the death of Ham and Steerforth. The sea is also used to give us insight into different characters, most importantly when the sea washes the corpse of Steerforth up on the shore, which symbolises the moral vacuum that characterised him, as he is treated like any other bit of wreckage that is moved around by the tide:
And on that part of it where she and I had looked for shells, two children--on that part of it where some lighter fragments of the old boat, blown down last night, had been scattered by the wind--among the ruins of the home he had wronged--I saw him lying with his head upon his arm, as I had often seen him lie at school.
If we think about the storm that appears in the last few chapters of teh novel, we can see that it symbolises the danger of ignoring the power of the sea and also signals the way in which the conflicts in the novel have reached their dangerous, tempestuous climax. Again and again, like death, the sea is shown to be beyond human control and power. For those who are dependent upon the sea, they ignore the dangers that the sea represents at their peril and must evince a healthy level of respect in order to survive.
Hopefully this will help you identify and comment upon other symbols in this novel.