What's a good way to improve on these three skills without having to go through a school?

I am looking forward to going back in the military as a reservist and if absolutely necessary full active duty. The job title I want is Public Affairs and I found out that they are looking for people who can write an article with as much details as possible or as brief as possible depending on what the ignment calls for. I want to improve on my spelling, writing comprehension, and punctuations. I don't want to take any cles cause I am already a full time student for Digital Marketing/Graphic Design. Does anyone have any Ideas on how to Improve on these three skills on my own? if you have any Ideas please list by subject if at all possible?