As we go about our days trying to get work done, we must communicate with many people using the written medium. These people may be coworkers, bosses or clients, and surely you don't want to put them off by making random grammatical or spelling mistakes.
English is not our native tongue, and it is normal for you to not be perfect at it. But you must teach yourself the rules of the language so that you don't end up making typos in emails you send to a prospective employer. Because unlike instant messaging, email doesn't give you the option to just send another message with an asterisk correcting the typo. The key to getting everything right is to always double check what you have written. If you need extra help, just run a spelling and grammar check with the help of a software. This is applicable to all forms of written communication including reports and assignments.
So, how do you teach yourself grammar? Firstly, read a lot. Read whatever you can get your hands on, and pay attention to how the writers use punctuations and how they use certain words. Next, read blogs or books on proper business communication, and try to follow those tips the next time you write something for work.
Still, if you ever end up making a little typo, don't feel embarrassed. Just make it a point to never make the same mistake again. Read more, write more; and with enough practice, you will find yourself becoming an expert.