I wish you luck with starting a union. SEIU is a good place to start because they have organizers whose job is to get unions rolling.
I'm in a statewide union that covers the RNs in California. The LVNs, CNAs, housekeepers and others in our hospital are represented by SEIU. They have never had any complaints that I've heard. It's not like goons in pinstriped suits come around to collect dues money, or tell you "You can't empty that trash can in the patient's room -- union rules say only housekeeping is allowed to do that!" They negotiate contracts, which is good, because the union local reps (who are your co-workers, not outsiders) have specialized knowledge on how to do that stuff.
Here's the bad side: it will be tough. Be REALLY committed to what you're doing if you do it. I've read a lot about union drives in other places. Management will resist as much as they can. They are not legally allowed to fire people who try to organize a union, but they will try to do it anyway. They will subject you to company sponsored "information sessions" until you want to puke. Your co-workers will SAY they want a union, but when it comes down to standing up for it, they'll be like the Disciples after the Romans grabbed Jesus. You'll hear all sorts of stuff from people who are philosophically opposed to unions, or had a brother-in-law who was in a union that was corrupt and sold out the workers.
If you DO get a union approved, which will take a year or more of struggle, then you have to get a contract. Management will stall for years, freezing your pay because "the salary scale is under negotiation." They will raise the pay of non-union workers. What management hopes is that enough pro-union staff will quit or get discouraged so that they can have a vote to de-certify the union. There's a whole industry devoted to helping businesses break union drives.
Is that scary or what? I hope you go for it, because we are working class people, and we need to stick together against the bosses who would pay us 50 cents an hour if they could get away with it. Even if you fail, you will get a great education in how the real world works. You will not be the same person you are now when it's all over.