I usually start out allying Hamburg and Bremen. They may be small, but they can swing pretty well for their size, and you already start with a bit of trust with them so it’s easier to call them into Wars, not to mention they will stay in your trade league a lot easier. Diplomatically invite Riga and Frankfurt into your trade League to get those important centers of trade. All of that should get you a solid economical base to work from.
Take “End the Sound toll” as your first mission. You want to make sure to get that bonus, as it solidifies your hold on the node, regardless of the other provinces you hold in the Node.
Next is you will want to get a larger ally, not so much to fight your wars for you, but as a deterrent from being pushed around by someone bigger than you. I generally suggest Poland or Bohemia, leaning towards Bohemia. Poland is powerful, but you will want to secure Danzig and Memel which will eventually bring you to conflict with them.
As far as expansion goes, you will need to be very opportunistic and selective on your wars of conquest. Your first target will likely be Mecklenberg. It holds the remaining provinces of your capital state (which you will want) and will provide you with a solid base to work from. Luckily, Mecklenberg has a habit of allying the Teuton. If this is the case, try to time your attack to coincide with the Polish invasion of TO. If you are lucky, you will be able to occupy Mecklenberg and at the very least get to Danzig (and Memel if you are *really* lucky, but don’t count on it) before the Poles. Danzig is very crucial, and going tall will make it difficult for you to ever challenge Poland, so this is your main chance. From there, you can expand as you want just like any medium power in the Empire.
On Danzig: you will probably want to take it for yourself. Put a trade post on it, and you will control the Baltic trade node, simple as that. Plus there is a unique event to Lubeck for when you control that province. If you are lucky enough to get Memel, you can either State it or release it as a custom trade city.
For Denmark: wait until the Swedish war for independence. The Danish armies will get tied up the Scandinavia, and their navies will likely either be busy blockading Sweden or hiding from your combined fleets. This is your best opportunity to end the sound toll. It may be a better idea to complete the mission and release Holstein for future conquest, then declaring another war after Denmark gets ravaged by Sweden again in the future.
For Ideas: Needless to say you will want economic, trade, and plutocratic ideas. Quantity is a good idea set too to give you the numbers you lack, and the quantity-economic policy gives you another 20% Dev bonus. From there you can colonize or build up your Navy, but you will definitely want to increase your Navy size as they will be your prime tool for controlling the North Sea and Neva.
For Religion: Swap to either Protestant (for the development bonus) or Reformed (for the trade bonus). It’s up to you on what you feel would be more helpful. Just make sure to have at least 50 prestige when you do so, or your trade League will break upon your conversion.
And that’s about it; at this point, you should be wealthy enough to merc spam and do whatever you want.

Hope this helps