Solitary witches have a challenge. Obviously you are supposed to learn as much as you can, but what exactly are you supposed to learn? Solitaries don't have a coven curriculum to run by, or even a teacher to guide them; they have to make their own decisions as to what they want to study.
Solitary witches don't have anyone to prove themselves to, except themselves. It's difficult, but also rewarding. If you need help, start looking around your community for those hidden resourses - occult stores and the like. A few lessons with an experienced person may just give you the confidence to keep going.
The list of things I'm giving you here is based on one from Covencraft: Witchcraft for Three or More by Amber K. Don't take it as gospel - rather, use it to inspire your own education.
Elements and Correspondances
Earth: Foundations, Body, Prosperity, the Material World Fire: Energy, Will, Purpose, Passion Water: Emotion, Feeling, Intuition Air: Mind, Intellect, Imagination Spirit: Balance, Creation, the All
Components of Ritual (Overview) Self-Preparation Setting Up the Altar Attunement/Centering Cleansing and Casting Invocation/Evocation Offerings Ending Your Rites
Why Use Ritual Tools? Your Book of Shadows Ritual Blade Wand Chalice Pentacle Other Tools
These are some more advanced topics, which might be best explored once the previous topics are well-understood. I would recommend finding a teacher at this point, but they can be explored on their own.
Tarot Runes Lithomancy (casting stones) Pendulums (radiesthesia) Geomancy Scyring
((There are many more; these are just a few))
Discovering Your Talents
Telepathy Precognition Levitation Clairvoyance Clairaudience Telekinesis Teleportation Channeling Remote Healing
Spell-casting Trancework Psychic Self-Defense Candle Magick Cord Magick Amulets and Talismans Magickal Names Chanting Drumming Circle Dancing
More Advanced Magic
Chakra Work Aspecting Deity Past Life Regressions Astral Travel Shamanic Journeys Invisibility Shapeshifting Artificial Elements
Power Animals Plant Devas Herb Magic Weather Magic Tree Magic Stone Magic Ley Lines
Herbal Healing Psychic Healing Flower Remedies Massage Nutrition Remote Healing Visualization Reiki Chakras, Energy Work Gemstone Healing
This is only a list of suggestions; it by no means covers everything. It is designed only to give you idea to things you would like to expore yourself. You can never read too many books or ask too many questions! As well, once you start understanding most of these things, feel free to start creating your own spells and rituals. Once you understand how the components of ritual work together, you can create your own that will be that much more meaningful to you.
The Internet can be a good resource - as long as you make sure you question what you read there. If something doesn't jive with you, chances are there's something not right about it. Think critically, and don't believe everything you're told as absolute truth and you'll be fine online.