You have now done all of the hard work learning about all of the different techniques involved in academic writing - don't let yourself down at the last hurdle by not ensuring that your spelling and grammar is 100% correct.
1. Even if you are sure that your essay is word perfect - get at least one other person to read it. A paragraph that makes complete sense to you may be confusing to someone else. Ideally, ask someone who understands your subject to proof read your work.
2. If you can't find someone to do this, then ask someone who doesn't know your subject - they will be able to pick up on the little spelling and grammatical errors that you have missed.
Ask someone to proof read your work and then make any necessary changes before submitting your work.