In EM when you right click on a table there are many options available to you.
Try all these options out to see what they do. Of course Delete is not a good idea.
But what you want to do is go to the Design Table Option. It will open the table definition in design view, and from there you can add a column to the end of the list, and specify a data type, or you can right click and insert a column between 2 others.
I would also suggest you lear the t-sql equivelent way of doing these tasks.
Look thru books online, or purchase a book like Learn t-sql in 21 days. Not the best book ever, but good for beginners