Well, first would be squaring up your quilt top. I would put it in the washing machine, hand-wash cycle/gentle cycle, and only allow minimal agitation if any at all. After the top is saturated, spin all of the water out, then carefully remove it from the washer. Spread it out on a table, the floor, or a wall, pulling it (gently) tight, or until it has no wrinkles or loose spots. If possible, attach it to the surface you have it stretched out on, for example pin it to the carpet or tape it to the wall or table. Leave it in place until it is completely dry; aiming a fan at the top will aid in the drying process. After it is dry, place a straight pin in each corner, so they each look like a nail partially hammered into a board, each positioned in each "corner of the square". Just as a builder squares a foundation of a building with sticks & string, wrap a string or thread around each pin going from one to the next. There are a couple of different ways to mark these lines. You can mark them with a wash out pen, fabric marker, or place talc or cinnamon on your thread & pop them like a chalk line. Either way you choose will give you your cutting line.
As far as mitering your border, after you square your top, sew on each of your border strips, leaving more legnth than the width of your border strip. For example, a 6 inch border, I would recommend a 7 inch "tail" on each end, and stop sewing 1/4 inch from each end of your top. (I usually take one of my rulers before I begin sewing & mark the 1/4 inch start & stop point of each corner.) This is the important step. If you do not leave the 1/4 inch at each corner your miter will not lie flat, but instead be puckered. I will refer you to a couple of links with pictures for the remainder of the process. There are a couple of ways to choose from, & the pictures will show it better than I can explain it so you will understand.
http://www.geocities.com/quiltfrenzy/miter.html
http://www.frommarti.com/miter.html
http://www.kayewood.com/tips.html
I hope what I have said helps, and makes sense. There are many ways to do either of these tasks, so I'm sure someone else here will be able to offer a more simple method.
Good luck!