as a starter, play at the slowest speed possible at which you can play every single note perfectly. if you cannot follow the correct tempo as a result, do not worry too much at this point since you're learning to sightread. when you're given a music, play every single note correctly, before you know the next sets of notes you're going to play, don't press the keys yet, press the keys until you know it'll be perfect... and take AS MUCH TIME as you need for preparing to play the next set of notes... that's what you need to practicing b4 you start sightreading other music... it's a very good habit... also, it's good to know your scales very well, because music is built upon it...
the sight reading part, take one fairly difficult music (harder than you can usually play) from each period follow the above step through the music several times, you'll start to get a feel of how to mactch the higher/lower-end notes to the keyboard, and how to apply the understanding of scales to the music and such. after several times, put these music away, pull some easier pieces out (those that are of your level), try the above steps, you'll find them much easier...
so... an example is, if you try to attempt to play the rach 3 conerto, once you get through the entire music (no matter how slowly, how poorly), when you put a chopin noctorne out afterwards, it's gonna feel much easier... true??