Needlepoint 101 : There are no rules - but there are some suggestions!
General
It is typically recommended to start with white or lightest colors to avoid "bleeding". Bleeding is when lighter color threads pull and blend with the darker colors
Traditionally, people like to start in the upper right hand corner, especially when stitching using the basketweave technique
Use a micron pen for outlining anything on the canvas, this avoids transferring ink (more "bleeding") onto your thread that happens when done with pencil / pen / sharpie
Mesh Sizes
13 Mesh - Holes of canvas are larger and require thicker thread
13 Mesh projects are good for beginners but small 18 mesh projects are also good for learning!
18 Mesh - Holes are smaller, typically helps create more detailed designs in smaller areas
Finishing
Once your project is complete, you'll want to turn it into something! This is called "Finishing"
You'll need to add extra rows on designs to help with ornament finishing.
You can "self finish" or go through an LNS to see if they offer Finishing (Prices can vary greatly from store to store)
Where to buy
You can buy threads like DMC Floss at popular craft stores like Micheals or you can find more unique fibers at a "LNS" aka Local Needlepoint Stores
Hours at these stores will vary greatly, this is common in most needlepoint stores - check online to make sure!