For shared flats the #1 resource is
- WG-gesucht (available in English and the link already points to Seligenstadt. Much of the content is in German only though and it works better in the major cities. You typically contact tenants already in the flat and convince them you would make a good flatmate. Some will insist on getting to know you in person, some will be OK with meeting first via Skype.)
- Facebook groups (search "WG" (Wohngemeinschaft = shared flat) + city name and you will find.)
For renting a private place
- immobilienscout24.de as suggested by @benroth.
- try to contact a local "Makler" to find you a place (at a fee that can be a few months' rent)
But in general, see whether your employer (or whoever makes you move to Seligenstadt) can be of help.
Regarding pitfalls, you should know and understand what you sign (ask for a translation or a friend you trust to double-check) and be aware of the usual pitfalls in renting apartments. Craigslist has a good list on avoiding scams, but it sure can be extended. On the sites mentioned above, if an offer looks too good to be true, it probably is a scam.
Even in legit situations, make sure the duration of contract and period of notice meet your needs.
Finally, why don't you consider a short stay solution first (Airbnb) and try to find a place once there, options will be better.