I've been wanting to go out in a nice skirt and top for a little while but I haven't worn one before and was wanting toknow if anyone had and advice on how to feel more comfortable doing so.

The first time I did that was incredibly hard, but because of the overwhelming need to dress properly in public I forced myself out the door and kept my head down and plowed ahead.

I will assume you don't mean physical comfort.  But for the physical side if there is any chafing, body glide works well   In terms of wearing a skirt in public I would practice the different mannerism when wearing a skirt or a dress.  Things like sitting down and getting up.  Getting in and out of a car.  The walk is a little different, a bit more free, but that varies a lot.  I would stay away from anything that is on the short side, until you get used to it.  Like any other change in public you can plan on making the venture out a short time period at the beginning.

I was thinking an ankle a line skirt for starters.  No way I could ever think about doing something short. My other issue is I'm still quite flat but have the hair and mannerism.  The other issue I have is I can't cross my legs without putting them to sleep so longer skirts and dresses are my only options

You don't need to cross your legs.  Even in a shorter skirt, just keeping your legs together is fine.

Yes, definitely keep your knees together when sitting in anything higher than your knee line. Avoid wearing them on windy days - even  the long ones can fly up. If it does get a bit gusty hold a side down with on hand, usually works. Keep in mind that some skirts have different practicalities and impracticalities - no one style is the same as the other. If it is a long dress be careful of getting it caught in doors or treading on it when using stairs. Pays to smooth it down at the back when you go to sit that way it'll be smooth back there when you stand up and not all crumpled. Be careful that it is not see through when you stand in front of a light source (like the sun) you might need a petticoat or slip. Be really careful not tuck it into the back of your knickers after you've been to the loo, can definitely happen.