Welcome to Mercenary's Guide to Blue Beetle Botlane!
Thank you for reading.
As a disclaimer, I am no pro player. Far from it. Some of my item suggestions might be a bit off and at a higher level. But Blue Beetle is my favorite champion due to the versatility of his kit and the heavy aggression is playstyle provides due to his hybrid classing as a Marksman/Assassin.

Build
Starting Items:
To start off with BB, it honestly comes down to personal preference. The best starts I have tired are:

1. Nil Weapon and Tier 3 Health Pots
Pros:
-Safest starting build
-a lot of sustain
-cheap stats
Cons:
-Builds into literally nothing

2. Coda and Tier 3 Health Pots
Pros:
-A lot of sustain
-Coda builds into a core item.
Cons:
-Requires more aggression
-Less safety with no bonus health.

3. Tier 2 Coda Blade
Pros:
-Mad Aggressive
Cons:
-No sustain. At all.

Obviously, the choice comes down to you. To new players, I recommend Nil + Tier 3, which is what I usually start with. If you want to build your starting items towards something though, Coda plus Tier 3 is also a great start.

Core Items:
As a dual Assassin/ADC, unlike other ADC's you don't build Attack speed as much, and focus on pure damage. This is why the first item I recommend getting is Coda Blade; with the modifier, its 50 cheap damage and can allow you to snowball. Following into deathstroke's claymore increases your burst. Huntress' crossbow provides dual AS/AD along with a decent passive. Joe Chill's is required on all ADC's for the insane damage it can provide. Velocity 9 is a personal preference but not required. Good for team comps where you can't go into Assassin form so often.

Offensive Items:
If you lean towards the ADC role, Shades Cane and Zeiss are always great picks. Soultaker Katana is definently not as potent as it was last patch but still provides decent sustain in a teamfight and a chunk of damage.

Defensive Items:
If you are getting bursted down in fights, Ras Al Ghul provides great all around stats along with a decent passive. Lobo's is great if your not too good at ADC, as it allows you to chase better while providing Health and Damage. Batman's Belt is kind of both offensive and Defensive; it provides a great chunk of health while giving you the ability to insta-gib squishies.

Situational:
If the enemy team is mainly AP, or your being targeted or laning against an AP threat, Beowolf is a good choice. If the enemy team is mainly AD, or your getting rekt by AD Champions, I recommend Suit of Sorrows. These are typically only picked up though if you know it can benefit you more then damage.

Stolen Powers
This is the part which I cannot be thorough. The main stolen powers I wield when I play are always Invulnerability and Super speed; two stolen powers I feel are essential. However, there are multiple great choices. Psychic Assault has use, along with X-Ray vision and if necessary, Cameras. This section is mainly up to you, but I threw my own two cents into this.

Build
Bread and Butter. Max Q first obviously; it is a huge source of your damage and provides a solid poke and waveclear. Between W and E, I don't know which should be maxed first. They are typically 'one and done' skills for me until after I max my passive. The passive is powerful. It allows you to out-trade enemy ADC's easily in lane. Don't underestimate it. I also grab a point in the Ult whenever possible for obvious reasons. There is a reason he is both an ADC and a Assassin! Make sure to make good use of all skills in both forms. This separate's the good Blue Beetles from the Great Blue Beetles. (Even though im neither)

Conclusion
While my knowledge of Blue Beetle is limited, he is my favorite champion and I hope he can become yours too. His Kit and gameplay is insanely fun and unlike anyone else in the game. Thank you for reading my guide.