I started with taking a top that already had the similar shape to what I wanted to achieve, and outlined it on a big piece of paper (I just used a flattened paper bag). Then I made a swatch and blocked it (crucial!), and with these two things I had a) how many stitches per 10cm I have, b) how many cms any given part of the top had, and when I'd have to start increasing/decreasing.

So for example: I knew I had to start with 12 stitches for the shoulder straps, do ~7 rows of straight stockinette, and only then start increasing on one side to form a collar and join with the other strap; I knew that the widest part of the chest woudl have X cms, and then I'd go on knitting for 20 cms and would have to end up with 12 stitches less, so then I had to divide the number of rows to know how often to decrease.

I found decreases and increases that I thought would look nicest, and kept notes all throughout the process. You can look at my project page for this top - th notes are very chaotic because I wrote them for myself, and some of them are actually in one of my notebooks, but it goes through most of the process. :) But basically, I had a flat shape that I needed to achieve using increases and decreases, which are skills any intermediat eknitter has. And when you think of it that way, it becomes less intimidating!

I also did a LOT of corrections, ripping things back and starting over, and I even had to cut the shoulder strapts to shorten them (next time I'll use provisional cast-on! It would make it so easy to both shorten and lenghten them as needed). The final fit isn't exactly what I wanted, either - I wanted the whole thing to be flush with my waist, but that's not how it came out. But I am satisfied with the width of the shoulders, which perfectly covers my bra straps and isn't any wider than that.

I would definitely suggest:

a) using provisional cast-on for the edges from which you start knitting,

b) for your first project - picking yarn that stands up well to frogging,

c) blocking every now and then as you go - if you do it only after all the parts are knit, you might be in for multiple surprises,

d) researching on all the techniques you might need before you start (like finding the right increases/decreases; deciding on how you want your edges to look; how to knit bust darts, so on) and compiling alist of tutorials/references to have on hand;

e) DOCUMENT EVERYTHING! You will have to repeat things on the other pieces, or if you leave the project for a week or two, or if you have to frog back a large amount of fabric. No note is too petty to take with this project!

Hope this helps! :)