First - learn your position "CAGED Scales" in all 12 keys. This provides you with a quick reference as to where you can locate notes and what position is best suited for a specific single note passage. Obviously with classical guitar repertoire we are playing much more than single note lines, but this is a great place to start. Take single note melodies in different keys and play them on the guitar (doesn't have to be guitar music). Through this method, you'll learn to recognize (for instance) - that if you are playing in G major, the "good places" are: Open Position, 2nd Position, 4th Position, 7th Position and 9th Position (depending on the range and note choices -- obviously, lower note ranger would make sense to play in lower positions/etc...).
Second - learn how to melodically and harmonically play all your intervals (m/M 2nds, 3rds, 6ths, 7ths; d/P/A 4ths, 5ths, Octaves and Unisons). Know what the shapes look like cross string, and the distance on a single string. Expand from this into learning to identify common chordal patterns on the guitar. If you see a chord shape on the guitar that's made up of a m3rd and a P4th you'll be able to recognize that that "shape" is like the 4th through 2nd strings of a C-type chord (EGC or 3rd, 5th Root voicing). If this voicing is in the key of E, you'll know you can play this voicing easily in 4th position (G# on the 4th string, B on the 3rd string, and E on the 2nd string. A good starting place if you haven't done this is a method like the Carcassi, Carulli, Sor, etc... Methods. While the teaching of technique is all over the place, it's a great resource for learning the different "guitar friendly keys" and seeing commonly used chord voicings/etc... The methods usually focus on 1st position (more specifically the first 5 frets of the guitar) when working through the keys, but it's a good place to start.
From there, find sheet music to read through. Looking for textures like (scales, melodic and harmonic arpeggios, chords, etc...) to help you understand what you'll be playing and sight read a little every day. ALWAYS pick something new so you don't have a chance to "remember" what you played the day before. The complete works of Tarrega on this site are great, plus the lower volumes of the Delcamp method are great too. Anthologies of pieces work well for this purpose as well.
Nick Cutroneo - Classical Guitarist, performer/teacher/suzuki instructor