Two books deserve your attention:
Rocket Propulsion Elements, George P Sutton; earliest editions are
simpler to study, and have all the essentials, IMO. I favor the second
edition, personally.
[NASA SP-125] Design of Liquid Propellant Rocket Engines (NASA Space
Vehicle Design Criteria) is quite comprehensive, and is available free
as PDF online.
Reading everything by Robert Goddard is likewise prudent, starting
with A Method of Reaching Extreme Altitudes.
I started at Aerojet-General at age 17, in 1957, as a result of a
small liquid rocket I built and entered in a science fair. Today, I'd
personally focus on SpaceX, or perhaps a small niche company that has
a good track record.
This aRocket forum represents the best of the best among both pros and
amateurs. Inquire here for good, responsible replies.
Get our asses to Mars, Charlie. We're counting on you! All the best in
your future!
Edward