I am making a small project for myself but have very limited knowledge on electrical engineering (V=IR is about all i know). I want to power nine LEDs (max 28 mA, 2.25V) with one battery. Do i connect them in series or in parallel? What voltage battery should i use? do i need a resistor? I have no clue how to wire this all up and with what components, but im sure some of you electrical engineers could help.
You can connect LEDs in series or in parallel, or in a mixture of the two. You can calculate the required power supply characteristics from the LED's forward voltage and current (in the data sheet for the specific LEDs)
it depends what L.E.D.s you are using. if they are 5mm red ones they would need less power, if they are Blue one they would need more power. and if they are 1 Watt based, they would be more power hungry.
you can try www.ledcalculator.net or www.hobby-hour.com/electronics/ledcalc.php to calculate what works out best for you.