First, have a look here at this article here in the Articles section of ETO:
https://www.electro-tech-online.com/articles/the-basics-of-bypass-capacitors.513/
i know they can hold charge and release it instantly, i also know they charge expotentially
The rate at which a capacitor charges and discharges depends on the capacitance and the resistance of the circuit.
Charge and discharge are both exponential (note spelling), hold that thought about releasing energy "instantly", it may be very quick but not instant.
i read they can block certain frequencies but then why would they be used on a dc circuit(no waveform)?
The reactance of a capacitor decreases with increasing frequency, the effect that will have on the circuit depends on how the capacitor is connected in the circuit. The statement "block certain frequencies" is oversimplified to the point of being wrong and misleading.
Even in a DC circuit there are "waveforms", what about at switch-on time? There is a great big step change in voltage.
There can also be noise in the circuit, noise picked up from various electro-magnetic radiation (radios, mains wiring etc), and if you are thinking about very small signals, there is thermal noise due to the little electrons all jiggling about in the wires. (Tricky little devils they are!).
how do people know what size cap to use what type (electrlytic or not), what it should be made from
For coupling and de-coupling, capacitors are chosen so that their reactance is "low" at the lowest frequency you are interested in. A very broad and almost meaningless statement, but often there are no hard and fast rules.
However for timing circuits and tuned circuits, the capacitance value usually is quite critical.
As a generalisation, use electrolytics when the capacitance is above 1uF. Electrolytics are usually aluminium type, but tantalum can be usefull for small physical sizes. When it comes to tantalum, some people swear by them, some people swear at them. In some areas they have been rather unreliable.
Below 1uF down to a few 100pF polyester or multilayer ceramic are ok.
Low value capacitors are often single plate ceramic.
Does this help?
JimB