It depends on the type of leukaemia. In acute leukaemia (myeloid/lymphoid), symptoms are usually related to marrow failure. So the patient may have tiredness fatigue, breathlessness (anaemia), infections (neutropenia) or bleeding (thrombocytopenia). Sometimes patients may present with tissue/organ infiltration in which case the symptoms may be related to the organ involved. Whereas in chronic leukaemia (myeloid/lymphoid) the clinical presentations may vary enormously depending on the stage. It may be totally asymptomatic in early disease but in advanced disease the manifestations are usually related to the organ involved (marrow failure, hepatosplenomegaly, lymphadenopathy, other tissue infiltrations). I suggest you have a look at a standard text book in Haematology (eg. Essential Haematology by Hoffbrand and Lewis