I spend a fair amount of time in Germany, and indeed have German satellite TV, so I already follow Borussia Moenchengladbach and Hamburger SV. That's mainly down to me spending most time in the lower Rheinland and Hamburg regions.
Borussia M'Gladbach (usually known as 'Gladbach' to stop using too many digits) are incredibly well supported for a town about the size of Reading. They regularly get 50,000. You see their club flag everywhere in the small towns and villages up to the Dutch border. They won the league several times in the 1970's and have been threatening since to bounce back. They play decent attacking football and the atmosphere inside the Borussia Park is one of the best I've come across. Like all German clubs, having a beer and sausage around the stadium all part of the matchday ritual.
Hamburger SV are really a first divsion club now trying to get re-promoted. They won the European Cup many years ago and with Hamburg being a large port city, they still get crowds over 45,000 in the 2nd Division. The club has been incredibly poorly managed, and get through managers and directors like there's no tomorrow.
Here are some snapshots of the other runners and riders.
Bayern Munich are the Man U of Germany. I have never met a non-Bayern fan who likes them. Unfortunately they have the money and resources, and sadly it has to be said some dodgy business people, one recently spending time behind bars, but it seems every season, they are the team to beat.
Borussia Dortmund and Schalke are the 2 traditional Ruhr district rivals which is the closest you'll get to the Black Country. Footballwise Dortmund have clearly been the better in recent years. Its fair to say they are some of the most fanatical fans in the country. Some will say because there are not alot of other attractions in the Ruhr district. Its got its own heavy industrial charm if you're into old mine shafts and steel foundries. Having been out in Dortmund several times, probably like a weekend in Walsall.
Hertha and Union Berlin are the 2 Berlin clubs. Hertha was west and Union east when the wall was there. Hertha has the imposing Olympic Stadium famous from the Olympics in 1930's. Seems like it should be a big European club, but they have never quite lived up to their potential.
Vfl Wolfsburg are also known as Wolves, so probably a reason for some to hate them. They are actually quite a different club, with alot more of a modern image due to their close association with Volkswagen. In fact, I've never come across a club so associated with their sponsor. The Volkswagen name is everywhere you look.
Bayer Leverkusen is another club closely associated with their sponsors Bayer pharma company. They have a quite decent track record in the Bundesliga, but what puzzles me is that if you visit Cologne (about 10 miles away), its all FC Koln in the bars, cafes, flags etc. You never see anything to do with Leverkusen.
FC Koln have a fanatical support, all part of Cologne being the Carnaval city No.1 , even out of Carnaval season. During Carnaval the fans come in fancy dress. They have one of the best club songs to the tune of Loch Lomond.
https://www.youtube.com/watch?v=hlZOj_vyYl4Eintracht Frankfurt have the advantage of being a one club city so they get crowds of 50,000 plus. They got to the famous final in 1960 at Hampden where they were beaten by Real Madrid. They usually finish top half and are one of those clubs that could go on to be big again.
Red Bull Leipzig are pushing Bayern at the moment but almost as universally disliked. They are seen as a plastic club created by Red Bull. They are actually not allowed to be called Red Bull as the sponsor's name is not allowed in the title , so officially its Rasen Ballsport.
Back in the north, Hamburger SV's traditional rivals are
Werder Bremen who have the bragging rights now due to being in the First League, but they could easily swap places next season. Werder are a well supported club in the region and a long tradition of being in the top flight, but have never quite had the resources to compete at the very top.
The other Hamburg club are
St Pauli. They are also in the 2nd League, where they have spent most of their existance. They are a complete one off club with a wacky, political left wing image. After a big demo in the city, police were moving on demonstrators from sleeping in parks, so the Pauli Director allowed them to kip out on the pitch. As the club is situated in the red-light district with its many bars and dives, players were always getting caught out on all-night benders etc, but that's going back a few years.
Anyway, hope thats of some help if you want to cheer on one or other of the teams.