One of, if not the most, iconic Bonsai species. A slow grower with many unique needs, and nearly overwhelming amounts of information online.
Most bonsai should live outside, and you should select species that can thrive in your region. While many people want an indoor bonsai display, these are often only temporary setups. Except for tropical species and succulents, the majority of bonsai species require a seasonal cycle and the cold of winter. The primary killer of trees for beginners is keeping their new Juniper, Maple, or Pine inside.
Bonsai need a lot of light, typically far more than an indoor location can provide. The majority of species will do best in as much sun as you can give them, protecting them from the hottest summer afternoons with shade cloth. Most of the rest of the trees will want slightly more protection and less hours of sun than that, but few bonsai will prefer a mostly shaded environment.
Protect the root system from sustained freezing temperatures. Move trees into an unheated garage, shed, or cold frame.
There is a wealth of conflicting or at least overlapping information on substrate, with many people finding working solutions. The truest answer seems to be, the best substrate likely depends on your local environment, the amount of sun, humidity, and wind that you get affects how quickly it dries out. Importing expensive substrate is likely not necessary, and finding what you can acquire locally will save money.
Developing trees with growing or large root masses focused on vigorous growth often need large pots, and costs of quality substrate can be high. It is often sufficient to use more regular potting soil mixed with inorganic material such as pumice or perlite (1:1). A popular, affordable mix of simple ingredients is pine bark and perlite.
The most traditional bonsai mix is likely equal parts Akadama, Pumice, and Lava Rock. You can mix some organic material in to retain more moisture if needed based on your environment or the species. Getting quality Akadama can be expensive, and many people are experimenting with mixtures using more local ingredients.
Repot right as the buds begin to swell but before new leaves and needles unfurl. Comb out old soil and trim back circling roots.
One of the most fundamental ideas to keep in mind working on all trees in all phases is Energy Distribution. Trees have varying amounts of energy at different times of year. They are either storing or expending energy. It can be directed towards one location or distributed throughout the tree. It can be unbalanced, with some random shoots growing long and other branches having no growth. Many species are 'apically dominated' which means they want their energy to go into making them a tall tree, with very little naturally going towards the bottom branches. Others are 'basally dominated' with the opposite effect.
Early on, energy is usually targeted to fewer locations, often times just one. This is done with leaders and sacrifice branches, and other growth points are removed. It is rare to see an amateur with a bonsai tree that has a 5 foot long branch on it, but experienced bonsai artists will. A leader or sacrifice branch will thicken the trunk from the point it is attached and below. So often they are not at the apex of the tree, but lower down, to help build taper. It is also common to grow a very tall leader for several years, and then eventually chopping the tree down low, right above a new growth point, to allow that to take over as leader and create taper through repetition of this step.
As the tree matures, rather than focusing energy, the goal becomes distributing it. By spreading the energy equally across all branches, you can reduce inter-node and leaf size, and prevent branches from thickening too much, requiring them to be removed and regrown. To do this, shoots are pruned to balance them, and often lower branches are allowed to be stronger than apex growth to fight a species' natural tendency to grow tall. You may still be using leaders or sacrifice branches in phase 2, but not in such an extreme way.
Phase 3 trees will be entirely focused on maintaining health and evenly distributing enough energy to get fine ramification, but not grow out of control. But you cannot stop a tree from growing, and should not try. If a branch becomes too thick, or energy becomes unbalanced, perhaps you need to make a decision and try and regrow an area. Remember that it is possible to go back a stage with bonsai, they are not hard and fast ages, but instead reflect your current goals for the tree.
Decandling is a pruning technique that leaves the new growth of needles (called candles) until they begin to split, then removing some amount of them. By default, leave about 1/3 of the candle behind. For young trees focused on growth, decandling should not be done, as it halts elongation and thickening.
Standard baseline rule: Water thoroughly when the topsoil looks slightly dry. Never let the rootball dry out completely.
There are many types of fertilizer including solid and liquid, organic and chemical. There are many intricacies and pros and cons, but at the end of the day, fertilizer is essential to bonsai. Fertilizer is not medicine, or food. It is more like vitamins. It will not fix a sick tree, and in fact, you should avoid fertilizing trees that are struggling or have recently had a lot of work done to allow them to recover.
Fertilize heavily to encourage vigorous growth. In this phase, you are building thickness, roots, cutting back and regrowing. You will see larger leaves and longer inter-nodes, neither of which should concern you as most growth will not make it into the final tree.
Fertilize a regular amount during growing season to help develop strong branches. Too much fertilizer/vigor can result in longer inter-nodes which is becoming less desirable.
A mature tree in refinement still requires fertilizer to stay healthy, but in lower quantities to keep inter-nodes short and leaves smaller, which are desirable qualities at this point.