Important to read the instructions first and work out the amount of ease eg stretch t shirts have negative ease and the finished measurements will be smaller than actual body measurements. Woven fabrics normally have positive ease and finished measurements allow for movement ie bending over etc
To accurately work out the pattern size, you need to compare both
I love stylearc patterns, they are amazingly well drafted and very up to date but not for a complete beginner. Helens closet eg the ashton top is a lovely pattern and a good intro to sewing that does not need much knowledge. HC patterns are very hand holding
ITS is in my favourites, love notions too. I have many designers in my inventory. Bust sizes are not bra sizes. eg over bust is 38, full bust with bra is 40. 40-38 = 2" which is cup size B. Cup size C is 3" etc. Many designers offer different cup sizes, as different pieces and it certainly saves the angst of doing a full bust adjustment
Pattern emporium, Tessuti, I have many favourite companies but I would suggest to get used to one designer to start with and beware of different seam allowances. Some use 3/8 inch and some 5/8 inch. I prefer 3/8 which is 1cm
Sidlined, never a professional sewist. The love and care would never be cost effective. I have been sewing for 69 years and started on a dining table like most. Now, on my own, I have a fully equipped sewing room, my haven
Right now I am working on a Helens closet pattern, yanta overalls and the instructions are wonderful