Minnie's skirt is traditionally a circle skirt. This is the best time to buy fabric for Minnie costumes, as it is in stores for Halloween. (Minnie's dot pattern is known as "coin-dot", and it is hard to find in stores at any other time of year because it isn't popular in street fashion right now.)
If you do not have a pr. of pinking shears, buy some (unless your machine is a serger); cutting with PS will keep the fabric from raveling on you while you sew, and afterward, which is especially important when your pieces are cut on the bias.
A circle skirt does not involve gathers, but it does take more fabric, which is an issue is money is tight. (Time/difficulty vs. money). Sign up for fabric store loyalty programs so that you get coupons in the mail. (It shouldn't be much of a problem with a small size, but as the sizes go up, bias-cut designs do take more fabric.)
On a circle skirt, the trickiest issue will be the waistband. As the skirt is cut on the bias, you will have to be careful that the cut edge does not either stretch or ravel at the waistband before you put the band on (you can do what is known as "stay-stitching" to prevent that.) Also, elastic is not a suitable waistband for a beginner on a circle skirt, so you will need to have some kind of opening at the back or side in order to get the skirt on or off so that it won't be too large at the waist. As you are not very skilled yet, I suggest using a large seam allowance and either Velcro or a snap-strip to do that; zippers are rather tricky for a beginner. (Or, you could just put a hook & eye at the waist and leave the rest of the placket open, putting a pr. of matching solid playground shorts underneath.)