Here are some guidelines to follow, and topics to study.
Ngons are faces with 5 or more edges. For now, you can consider them bad. Keeping your mesh as close to all 4-sided faces as possible is ideal! Also, if import a mesh with NGons into a game engine, the engine will automatically triangulate your mesh however it chooses. You will have more control by avoiding Ngons all together. Most your Ngons are being created by stopped edge loops.
2. Modeling in Quads
Most people will tell you to avoid unnecessary triangles. This allows your mesh to animate and subdivide better in the future. I've marked some edges that you can delete in red. I've marked an acceptable tri
with green. This tri isn't creating a pole or star and is helping to reduce stretching of its neighboring 4-sided face.
3. Poles & Stars
A pole is a vertex that has 5 or more edges coming from it (a star has exclusively 5). If you were to subdivide, you would see an ugly bump where the pole is. Stars aren't as bad as poles and are sometimes unavoidable, but be sure to eliminate them where you can.
4. Resolve Edge Loops
You are cutting some of your edge loops short. Make sure an edge loop goes somewhere and doesn't stop suddenly and create an NGon.
Watch this short video by The Guerrilla CG Project for better understanding:
Your tentacles and the nose have the best topology. It seems you tried to draw the teeth in with tris, but this is unnecessary seeing as you will paint them in photoshop. A better way to create the bumps on the top side of the head is to select a face > extrude > move up > scale down the new face to a point. This keeps you in quads and leaves you with a more sound mesh.
Good luck & keep up the good work!