Yes, you can come to Germany first (which is what I recommend) and then apply for your visa.
I received a permit for English teaching freelance in Frankfurt. I was here initially on a student visa and after one year I applied for an extention, or rather, a visa 'conversion' to work freelance.
I think since you do not have a permit at all you will have to do the following:
-Come to Germany
-Find a place to stay (hopefully with your boyfriend)
-Register ("Anmeldung") with the local authorities in the town where you will be living (your boyfriend can help with this process- it's standard procedure for anyone living in Germany- even for Germans). You might need to take a rental agreement or a letter from the landlord (if you're not the legal tenant on the rental agreement) allowing you to stay at that address. You might not need it to register but I you'll need it for the Aliens' Office.
-Obtain 2-3 letters (the more the better) from potential employers stating that they would like to give you work, including number of hours per month and earnings (approx). In my case, I had my employer state that they needed an native speaking "American English" teacher, just in case they wanted to deny me based on the fact that British, Irish (and Australian) citizens are given preference for work permits before Americans (based on recipricol country agreements).
-Letter from landlord or rental agreement. Showing monthly rent, square meters, etc. They use this with the employment letters to determine if you can support yourself. If you are living with you boyfriend, you might want to his proof of earning for the last 3 months. It could help, if they determine you won't earn enough. But basically, they want to make sure you won't become a charity case and apply for social help.
-Obtain
health insurance if you don't already have international coverage from elsewhere. You can get German coverage for non-citizens, but do your homework and check around. Prices vary considerably. Also compare the cost with coverage from your home country, someimtes that can be much cheaper than getting German insurance.
-Apply for a residence permit with permission to work freelance.
Documents you'll need: passport, extra passpost fotos, proof of health insurance, employer letters, city registration and rental agreement/permission from landlord.
You should get (up to) 3 years residence permit to stay and work (mine was only granted for 2 and a half because my passport will expire 6 months before the 3 year mark).
As soon as you get here, you should start arranging/collecting your paperwork and look for a job - you'll have 3 months on your tourist visa (depending on the allowance for your country of residence) before you have to apply for the permit. I waited until the last week of my tourist visa to go apply and got an extention to get all my paperwork in order. I think the standard extention is 1 to 3 months. She didn't say how long I had, just to get everything together and come back in "a month or so". For my initial study visa, we made like 4 trips to the Aliens' office because every time I thought I had everything, they would ask for something more. I don't know if this was on purpose (to make me jump through hoops) or honest oversightS (emphasis on the plural!:).
I strongly suggest taking your boyfriend when you apply. If you're German isn't good, they might not be as cooperative.
Best regards,
Jennifer in Frankfurt