Generally IMAC chromatography (using a coloumn carryng Nickel, cobalt or zinc) is the first tipical purification step for a protein that carry a HIstidine tag. Afterwards you can perform a 2nd purification step to further improve the protein purity and/or remove protein aggegates using a SEC or other approaches on the basis of the MW of the impurities.
You can find some tips for IMAC on my blog ProteoCools at the following links:
Definetly go with IMAC as Manuele discribed too. However, IMAC consits of many different methods that you can choose from (FPLC; column chromatography, batch spin, Magnetic beads).
None of them is known for being "better". It mostly comes down to personal preference and available lab-materials such as a FPLC machine for example.
IMAC followed by SEC (superdex200 is a good resin for 75kDa). If you see more than 20% non-target protein following elution from IMAC, then increase the binding buffer concentration of imidazole to reduce non-specific protein. There are additional methods, but this is the easiest starting point. This also assumes that your starting volume is reasonable, otherwise you would need to concentrate by TFF or other methods.