Note: I have been playing for 4 years, teaching myself, and 1 year with a teacher. I was planning to continue with lessons but my teacher has gone back to university

I need help with the following:
Sight reading
increasing dexterity/strength and speed in fingers... < main issue! my fingers feel stiff as hell when I play...

For sight reading just put on a metronome and practice playing different passages. If you mess up, don't stop, keep counting and don't loose your place. Do this every day for at least 20 minutes and you'll improve in no time. Depending on your level of reading, just go to the library and get some beginners stuff or some xmas carol books and play through them.

You must have heard of Hannon by now? If you don't have his book I highly recommend it, the exercises he developed were made specifically for what you are asking for. Along with Hannon, try this exercise:

With the right hand put 54321 on GABCD and with the left to the same starting with 1 on G an octave higher. Put on the metronome and press all the notes down on 1, on beats 2,3 and 4, start with finger 5 and play the notes together with both hands while your other fingers remain on the keyboard. Do this with every finger and make sure you are lifting your 4th and 5th fingers as high as you can. Keep going with all the fingers.

much appreciated