I'm a first-time home buyer, so I've had to pick up a lot of this on the way as well as frantically calling my dad when we were writing up an offer! Here are my best answers to your questions, and someone else can clarify.
So you are pre-approved, which only means that a lender gave your info a cursory look and said he would most likely loan you the money. Sellers like to see this, because it's better than nothing. None of this really matters (at least as far as my situation goes) until you find a house you put an offer on. After the offer is accepted, you have a certain period of time to find financing. This is when you would call around to different lenders (including the one you were preapproved by, if you want) and get good faith estimates. Obviously, you want to find the one who will cost you the least amount in stupid fees/closing costs, as well as lower interest rate. As far as I'm aware, you can't lock up an interest rate until you actually apply for the loan. At that point, if you have a nice lender, sometimes they will hang on to your app and when rates hit what they think is the "low" for the next little while, they will lock you in then. Rate locks usually last like 45 days, I think. Probably depends on the lender. Again, none of this really matters until you have a specific loan amount/found a house.
A broker is a real estate agent who through qualifying exams/money/etc., was able to open his/her own real estate agency (brokerage).
Hope this helps!
You should get a GFE within 3 days of your application taken & credit run - so if you are pre-approved then that "clock" has definitely started ticking. Has it been more than 3 days? If so then you should have a GFE already, but since you apparently do not have one your loan officer should be able to email/fax you a GFE on Monday for whatever the current rates are. Some lenders will let you lock in an interest rate on a "floating property" meaning you don't have to have an address in mind, but would have to have one within a certain amount of time... but the majority of lenders will require you have a property address, and in the event of a purchase, many will want you to have a fully signed purchase contract.
For the properties you found on your own, contact your real estate agent and ask for further info on the home (they can send you details from the MLS listing) and to set up an appointment to view it - it's as simple as that.