The umbrella policy is usually tied to homeowner's insurance. Check different insurance companies to see how they work it. But quick summary: unless you can get under your parent's umbrella policy if they have one, you might not be able to get an umbrella of your own till you buy a house.
Get the maximum liability coverage you can afford. The difference really isn't that much. $25,000 in liability coverage is nothing these days with all these new vehicles that cost way more than that. If you are at fault in an accident, the other driver can sue you for what your insurance doesn't cover, so don't try to save a few bucks by scrimping here. Go for at least $100,000/$300,000/$50,000 (your insurer will call it "100-300-50").
You will probably find that State Farm's rates are among the most reasonable at your age. They were for me as well. I received a quote of over $1,000 for six months' coverage for me driving a 1978 Toyota Corolla 1200 from a competitor back when I was in my early 20s in the mid-1980s. Over $2,000 a year for bare-bones coverage 20 years ago! It was based solely on my age and not my driving record, and the quote was for bare Virginia minimum liability and no collision coverage on the car. Needless to say, the little 1200 cc engine wasn't exactly a hot rod, so that wasn't a factor in the rate quote.
A few years later the same competitor still quoted me over $1,000 for six months. I've been with State Farm for over 20 years, needless to say, especially after seeing some of the shenanigans other insurers try to pull. Stay with SF long enough without doing anything dumb, and you will have guaranteed renewal regardless of your accident or driving record.
One more tip: get the highest deductible on collision that you think you would be able to pay if you are found at fault in an accident (say $250 or $500), but get a zero deductible for comprehensive. If you hit someone else, you should expect to pay something anyway (life lesson). If your car is damaged by fire, flood, or animal collisions, or is stolen, this is generally considered not your fault and the comprehensive coverage comes in. Why be out of pocket in that event? Hence the zero deductible, and the cost difference is peanuts. Food for thought . . .