What kind of bit should I use on a old broodmare that was once broke to ride?

I have a approx. 18 year old broodmare. The only reason we got her was because she was a month away from having a mule foal. Well it has been 3 years now... The guy we got her from had a friend that road her and thats all we know. I have saddled her and she has done great and now its time to figure out what bit to start with. I will be riding her in a round pen (that we just got sand put in (: ) I have been riding for 3 years and have just started training. I'm not sure what bit i should start off with or if i should start with a bosal...any info would be greatly appreciated. oh and I will be riding her western.