I have some advice
I'm fairly new to sewing, myself, having only sewn for about a year and a few months, but I've learned a LOT! And I wish I knew now what I did know then.
First of all, very rarely do people fit commercial patterns straight out of the package, much like people rarely fit into ready to wear clothes off the rack. So, be sure to measure yourself! Also, most pattern companies make garments for women to be of a B cup, so if you are any bigger in that area, you will have to adjust the pattern, otherwise try and see if they have the pattern in bigger cup sizes, sometimes
they do. I suggest http://simplicity.com/index.cfm?page=fitHelp_main.html
this website for fitting and correct pattern sizes, which in most cases are 2-4 sizes off of your retail clothing size.
For jackets and tops you want a good armhole fit, so if your bust exceeds the B cup margin, then try for the high bust measurement if they don't have the pattern in bigger cup sizes. You will have to adjust it to get the right fit for your brazziere area, but that's not difficult. But since you have a knit top there in your pattern, that wont be much of a problem if you plan on making the shirt view (the different ways you can make them are called views). The skirt can get a little tricky, but it's not hard.
So, all that being said, you're going to have to learn some sort of basic pattern adjustment throughout your sewing career. It's really not hard with the right tools and books/magazines to help you, it's just a little time consuming. The best part about it is , I've heard is that it gets easier with each peice you adjust. And really, it all depends on how far off the pattern is from you and what type of pattern you chose to make. If it has an elastic waistband or some such detail, you could get away with little to no alteration.
Also! Crucial - cutting pattern peices with your sharp fabric scissors is a big NO-NO if you want your scissors to last you a good while. I learned this the hard way, so now I cut my patterns out with little scissors like children use for paper, you know just normal small scissors for my small hands, and then I pin the pattern to the fabric and use my large fabric scissors to cut the fabric. And if you really get into sewing, some people use rotary cutters for this process. It goes much faster.
For multisized patterns, I've been told that you can save the extra sizes for the future, if you are worried about your shape changing, or if you plan on making it for someone else, that you can cut out along the biggest size (plus all the other sizes lines, you will see how they overlap and portrude where others dont) You don't want to cut any lines off if you want to keep the extra sizes. However, you can cut a snip through to the size you want, and fold back the unwated sizes and pin it down like that. I've started to do this.
And also, generally, yes, you do want to wash, preshrink and stretch your material before you sew it. Most people don't stretch it though. Just make sure when you buy the fabric, look on the bolt you choose, and on the end, look and see how you're supposed to wash and iron it. Then before you wash it, to prevent clogging and fraying, sew the ends of the fabric together. Zigzag works well. ALSO! Make sure you get the right needle size and thread size for the fabric, that can make a HUGE difference. For lighter fabrics, you want to use about an 11 needle, heavier there's sizes 14 and 16 needles. You also have to make sure you use sharp needles for woven fabrics, and blunt ones for knits. http://www.wwwearables.com/savvy/needles.htm
there's a site for that info.
I also recommend going to the library and checking out a beginner's sewing book, or just a general sewing info book. There are tons out there. Also Threads magazine just started a fitting series in the current issue, so if you care to, you can go to your fav magazine place and thumb through it or buy it.
Learning to sew straight, zigzag and curved lines can be difficult at first, so to start out, you can get some paper and draw some lines on it. Without threading the machine, try stitching through the paper on the lines you drew. Keep practicing that way so that you don't mess up the fabric you buy, also so you don't keep ripping out thread. And always "backstitch" and then sew forward. It would help you as well to find someone around you who sews. If you can't, you'll be able to teach yourself to sew! I did
Another thing - ALWAYS iron seams! It's called pressing, and that's all you do is press the seams open. It makes a WORLD of difference!
I've written a ton and a half here, so I'll leave it at that. Hope this helps, and try not to feel overwhelmed! Little steps
And here's one more link so that you can learn all about sewing basics before you leave the house http://www.taunton.com/threads/pages/th_feat_sewbas.asp
There's also a PDF I can send you on teaching yourself to sew if you'd like it, PM me.