Geometry: Start geometry by solving basic theorems and problems from Challenge and Thrill of pre-college mathematics . Focus more on proofs rather than the theorem itself.

Combinatorics: Don't neglect combinatorics!! It is not only important for Olympiads but also helps you to develop a mind for challenging real-life computer programming problems. Start basic topics like piegon hole principle, counting etc from Mathematical Circles.

Number Theory: Mathematical Circles is a very good book to start Number Theory itself. Practice topics divisibility, remaiders, Euclid's Algorithms etc.

Algebra: Although algebra for beginners doesn't cover a lot but you can find sufficient material to begin with in Challenge and Thrill of pre-college mathematics.

Practice Problems: The best resource to practice from at this stage is British Mathematics Olympiad-Stage 1. You won't find solutions to most the problems but the problems are simple (not easy!) and interesting.

Geometry: Geometry at this level involves solving more complex theorems to begin with. An excursion in Mathematics. is a concise handbook containing most of the important theorems. Once you cover this, you can move on to Euclidian Geometry in Mathematical Olympiad by Evan Chen. This book have topics ranging from theorems to important Lemmas. Covering this book allows you to bridge the gap between intermediate and advanced level.

Combinatorics: The next level of combinatorics syllabus could be achieved by covering topics from Problem Solving Strategies by Arthur Engel. At this stage, also start Combinatorics chapters by Pranav Sriram (google search them).

Number Theory: lemmas are more important at this stage than theorems. Mathematical Olympiad Challenges is a good combination of problems and lemmas.

Algebra: Mathematical Olympiad Challenges and Problem Solving Strategies contains pretty good amount of material for Algebra at this level.

Practice Problems: Now, you can start solving British Mathematics Olympiad-Stage 2, Indian National Mathematical Olympiad, USAMO, and other national level olympiads. Solutions to most of the problems can be found on AOPS.

Geometry: to begin with, complete all the lemmas from Euclidian Geometry in Mathematical Olympiad by Evan Chen. Next, cover handouts by Yufei Zhao and start solving IMO Shortlist problems.

Combinatorics: start covering handouts by Po-Shen Loh. Also start solving IMO Shortlist.

Number Theory: handouts by Yufei Zhao for some important lemmas. Apart from this, start solving IMO Shortlist.

Algebra: Solving problems from IMO Shortlist is a very good strategy for practicing Algebra at this stage.

Practice Problems: You might have guessed it right! Practice IMO Shortlist to the extent possible.