Slipping the first stitch on each row will give you a neater edge. Great for scarfs, sweaters, etc...
It doesn't matter if you don't, however, if a pattern calls for you to slip the first stitch on each row - my advice follow the pattern, especially if you are new to knitting as there may be a reason for the slipped stitch. The first time that I make something, I follow the pattern - the next time through I make changes if I want.
You don't need to add stitches when you are slipping - unless you are passing the slip stitched over as part of the pattern not a decrease in a pattern. This is seen in lace or openwork patterns and then the added stitch is usually a yarn over.
Always read through the pattern completly - sometimes this can be a challange to visualize what it will look like. This does get easier as you knit more and more. When in doubt knit a sample of the pattern - this gives you an idea of how it will look, and give you practice prior to knitting on the item.
Good luck and happy knitting :knitting: