Have you talked to a recruiter? They may be able to offer you better answers.
1. I'm going to be getting married to my girlfriend before I join (if I do), but she has a son from a different father, would he be covered by my benefits? - Yes, if your girlfriend has custody of him, and he resides with you for more than 6 months out of the year.
2. I also have a baby on the way, who would be born before I join. What kinds of benefits would be available to us? Healthcare is free once you enroll your family in Tricare Prime. I'm not sure what other type of benefits you are talking about.
3. I want to be a graphic designer when I get out of the military. Do you have any advice on what field/branch would be right for me? I have no idea
4. Would they move with me if I'm sent to another base in the country, or out of the country? Yes, the military will move you, and your family (as long as it isn't a dependent restricted tour). Make sure your orders stated authorized move of dependents.
5. Is military life similar to civilian? Like, get up in the morning, go to work, etc? How does it play out? Yes and no. Much depends on your branch, your specific job, your unit, etc. Technically, you are on the job 24/7. Your regular duty hours depend on your unit, but you work until your job is done. Sometimes you'll be training, working late nights, overnights, etc. You can be called at any moment. When they say jump, you say "how high?". You are always on the clock. That being said, during slow periods, most units will go in to work in the morning for PT, then start the duty day and get off in the evening.
6. I also want to earn a degree. Any info on this? It depends on the branch. I believe all branches encourage education these days. You may not have time to physically attend class, but there are programs designed to help you get your degree, even if it means taking twice as long and having to take online classes.
7. I really don't want to stay in too long. Is there any enlistment bonuses or penalties on a short term contract? There aren't any penalties. Bonuses generally are given when you commit to a certain amount of time. These time periods vary, as do the bonuses. Regardless, you will have a minimum 8 year committment. If you choose to enlist for 3 years, once you get out, the remaining 5 years you will be considered IRR, eligible to be called back as needed.