I would suggest that to get the best results, you would need to be deliberate about two things:
- how you prepare the surface
- how you choose the finish
For surface preparation, the first thing is to get the wood flat. You can buy it flat (rather than rough), you can take it to a commercial shop to get flattened, you can use a hand plane (lots of videos on YouTube) or machines (ditto).
If you go with a hand plane, you might be able to skip the next step, which is sanding. Again this needs to be more deliberate. E.g. start with 80 grit, move to 120 grit, then to 180 grit. Draw pencil lines across the entire board so that you can check your progress (i.e. when the lines disappear). Search on YouTube for “how to sand a board” :)
For the finish selection, I recommend you to read one of the many finish comparison posts on the internet, pick one finish and try to get better with it. My first pick was shellac — it looks good but is definitely not fool-proof. Something like a hardwax oil (e.g. Osmo) can be more fool-proof, although I am sure more experienced woodworkers might take an issue with this recommendation! (Note that neither of those areis outdoor-proof!!!)
This depends on where you live, but I would not necessarily trust the local DIY store to carry a good finish. Or good sandpaper for that matter. Check the many sandpaper comparison videos on YouTube. I like 3M Cubitron.
For ultimate results, the common recommendation seems to be to read this book: https://www.amazon.com/Understanding-Wood-Finishing-Comprehensive-Troubleshooting/dp/1565235665.