Hello,
You definitely need to take lesson prior to understand what you might have to buy to continue and even understand what you mean by next step.

Board measures in volume (liters) and width (cm). Volume is for float ability and width for stability, the bigger the more float ability we say (weight in kg + 20-40 for medium wind kind) and more for beginner. Usually the more volume, the more width...but this is not always true.
Board also depend on program slalom, free ride, wave, beginner....the hull and the outlines are very different...and as a consequence the behavior
Mast measures in meters or cm...from 3,5m (350cm) to 5m (500cm) and more...and some of the significant parameters are the flex curve and the reflex dynamics (material related spring effect). Some mast are RDM (reduced Diametre mast) and others a SDM (Standard Diametre Mast). This diameter have an influence on the sail behavior and most importantly on the mast foot size and the wishbone connection.
Sails measures in square meters and program. They also have 2 other steering parameters, the luff length (mast) and the wishbone length (cm or meters). Again he program can be slalom, course race, speed, wave, beginner, ....outlines, shapes and steering skills are very different
Wishbone measures in cm or meter. Basically, the material durability and stiffness is making the price. There is also a bit of the program but...this is mainly in sync with the sail boom length.
Fins measure in cm, and program and type of board to fin connection geometry
Mast foot and extender measure in length and diameter like masts.
The bigger and the longer board (Wally 300l - 3,5/3,8m) will allow you to be moved with very little wind and technique. They will nevertheless be completely outruled by modern beginner alround board as soon as the wind gets around 7-8knots, both in stability, speed and playfulness.
Most probably you will keep your beginner board for about 30-50 sailing hours...this can be a full season...
Next, you'll need to understand what you are attracted to, blasting fast and straight, carving deep and jumping, making acrobatics... And you'll most probably turn yourself towards a said freeride or bump& jump board, a bit narrower....I guess you'll increase your sail quiver first by taking bigger size to fly faster on your beginner board.
Be patient, and as said earlier, it is all about practice...I usually say that we don't windsurf when we want, we windsurf when we can....that is to say when wind and tide are making the playground .... Windsurfing requires a bit more time than kite to be enjoyed...
Have fun, sail a lot
Plettil