I think the stuff you've been doing already is technically called "piloting" which I understand to mean "navigation near land".
It sounds to me like you're off to a great start. When you're looking at your charts, are you ever trying to figure out your precise location (i.e. take a fix) using something other than GPS? I think this would be my next step in your position. It's an easy way to augment what you're doing now. Find an easy landmark and use a compass to take a bearing to it. Use a parallel ruler to draw a line on your chart oriented in the direction of the bearing; this is a line of position and you know that you're on that line. Do it again for another easy landmark/buoy/whatever and where the two lines of position is where you are -- this is a two-bearing fix.
This is an important thing to get used to because almost all advanced navigation is based on lines of position. It's also fun, and an easy thing to teach to others on your crew. There's also a lot you can learn from there on including more advanced types of fixes.
The other thing you can start practicing is dead reckoning. From your last fix, figure out what direction you've been intending to travel (your heading) and what your speed over water was. Plot lines on your chart indicating distance traveled, say, every hour. You can then learn to take into account current and leeway to estimate your course made good. You can then check your estimated
position that you got from dead reckoning against a two-bearing fix.
Long distance navigation will basically involve more advanced fixes, for example using celestial lines of position, coupled with dead reckoning.
Finally, the bible of navigation and piloting is The American Practical Navigator
by Nathaniel Bowditch.
Have a good time figuring out where you are, and welcome to SailNet!