My question involves criminal law for the state of: TEXAS

A friend and I went last minute Christmas shopping. We were at a Kholes and realized we didn't have much money left so we thought we'd shoplift the rest of our Christmas presents. We are both broke freshmen in college. I had never EVER stolen anything before (and never will again). I look back on it now and am really ashamed of what i did and feel like a guilty criminal because frankly, my actions were criminal.
anyways, i stole some mens cologne for my brother, slippers for my mom, and costume jewelry (pair of earrings and bracelet) for my sister. We made it out to the parking lot but the LP team ran after us and asked us to go inside. Immediately we cooperated and my friend broke down in tears. I remained calm until they totaled up what we had stolen. I had stolen $103 worth of things and she $143.
They called the cops and that is when i freaked. He came after an hour or so and said he had to arrest us for a Class B misdemeanor. We went to jail and had to SPEND THE NIGHT there. We didn't get to share a cell and i was terrified the entire night. We did not get dinner even after i told them i had not eaten and was really hungry and spent a total of 16 hours in there. We were extremely cooperative the whole time and bawling.
I have never been in any kind of trouble and have a 4.0 GPA. I am well aware that what I did was EXTREMELY STUPID.
They set my bail for $500 but qualified for 'pretrial services' which brings me to my first question:
What exactly is this 'pretrial service'? I know that I didn't have to pay the $500...BUT i do have to show up to the county jail in a couple of days with a $20 money order but thats all they told me. Will i eventually have to pay off the bail?

Second: Is this pretrail appointment thing the same date as my court date? They did not give me any information about my court date, but I know for sure I have to go to court.

Third: What is the difference between hiring my own attorney or having one appointed?

Fourth: Do I plead guilty or not guilty, even though CLEARLY I am guilty. Which plea is it easier for me to keep it off of my record

Fifth: I was planning on transferring to UT Austin and was confident of my acceptance because of my 4.0 GPA and heavy involvement in school. Will this record keep me from being accepted?

Sixth: Will the court take into account that I've never been in any kind of trouble and that I'm a good person whose involved in the community with good grades despite my STUPID STUPID one-time mistake?

ANY advice will help me and i will be extremely grateful to you all. I'm a giant stress-case right now as well as my best friend. She's in the same exact boat as myself. Spending a night in jail was literally the most traumatizing experience of my life. Picture a 4'11 tall girl whose just over 100 lbs who has never been in trouble suddenly in an orange jump suit starved for 16 hours. I thought I was aware of all the consequences when i decided to take the items i took but BOYYYYYYYYYY was i wrong.

OH also: Is it possible for me to get MORE jail time even though I already served some time? and will the judge take into account that i've already spent the night in jail and see just how regretful i am and lessen my sentence?