¶ Welcome to the 40% cult!
This is a little reference guide for keyboards that are widely available for those interested in plunging into 40% keyboards. This does assume some background knowledge from the wider mechanical keyboard space, including knowledge of basic QMK, switch preference, sourcing keycaps, etc.
What is 40%? Good question, nobody knows. Some say 4 rows or fewer, some say less than 50 keys. It is generally considered to be a broader classification for small keyboards (although some more folks may have more strict personal definitions). Please be aware that many 40% keyboards have different keycap sizes and layouts than are supported by traditional base kits so please make sure the keysets you are buying support the keyboard you are building 
¶ Choice 1: Row Stagger vs Ortholinear vs Columnar Stagger
Row staggered keyboards are exactly what you are used to. The rows are offset from each other (or staggered).
Ortholinear keyboards, on the other hand, are organized in a grid like fashion where all keys form 90deg corners with all other keys.
Columnar staggered keyboards are keyboards where the columns are offset from each other. This is also commonly referred to as "ergo" although, that is a relatively vague term. This is also primarily utilized on split keyboards.
Generally, for a first board, one would recommend row stagger or ortholinear to become accustomed to the reduction in keys. However columnar stagger keyboards are perfectly viable for a newcomer as well and offer other, unproven, ergonomic benefits.
¶ Choice 2: Width
Keyboards can come in many widths with 12U being a common width that gives all alphas and a column of modifier keys on either side of those alphas. A 10u board on the other hand is only as wide as the alphas. Going for something in the 12-13u range and at least 4 rows (cutting down to 3 rows on your first board may be difficult) is the most beginner friendly option.
¶ Choice 3: Budget
There are many keyboard budgets, ranging from inexpensive $30 kits (FR4 sandwiches) to full blown $400+ experiences. For your first board in the 40% form factor I would remain somewhat budget conscious in order to assess your preferences.
¶ Choice 4: Hotswap or soldered
Hotswap sockets make it easy to build and change switches. The down side being that they are a mechanical part and prone to wear and tear over time (inserting/removing switches repeatedly). Many more affordable options and high end custom will require you to solder switches. Make sure to pick one which fits your technical skills and DIY risk tolerance.
¶ Choice 5: Materials
Choice of case and plate material is important as it will impact sound and feel. Generally metal options will cost more than their FR4 and 3DP printed cousins. This one boils down mostly to preference and budget.
¶ Choice 6: New vs Used
Like any hobby, there is a thriving after market for 40% mechanical keyboards. Due to the fact that many designs are offered once as a group buy and not available to purchase again, going used is often your best bet for something specific that already exists. You can also sometimes get a better deal on something used (although not always, the market is always changing). Purchasing new is more difficult if you are not purchasing from an active group buy. There are some good kits and boards that are (or at least were) available as in stock from various vendors and are noted below. However, if you see something you like that is no longer in-stock, it never hurts to try to find a used unit in the 40% Discord!
Some of the good (widely available) options:
- Planck: Around $100 for an aluminum case and a hot swap PCB. Great option for those curious about ortholinear layouts. You may also find a variant on Drop.com.
- Minivan (and minivan adjacent, like Coffeevan): The mini-van layout is considered to be a great layout for those getting into 40%. Injection-molded cases and solder/hotswap PCBs are available at Coffee Break Keyboards.
- QAZ: Cheap, semi-in-stock option for the more daring of newcomers. Large variety of case options (3D printed and acrylic).
- Fruitbar: A friendly 40% option designed to be compatible with normal base kits! Requires Soldering.
- Prime E: Good case availability from vendors in the community, good layout for those interested in unibody split/ergo/alice style layouts.
- Corne A widely available introduction to column-stagger, Corne has enough keys as to not be too daunting for a newcomer, but still different enough to provide value over the pack.