what you wanna do is just get a crescent wrench and remove the petcock (the thing sticking out from the bottom of the fuel tank) and just tip the bike over on its side; you can drain the fuel quick and easy this way. screw the petcock in a little ways and put a little fresh gas up in the tank and shake the bike around a little bit and repeat; if the stuff draining out is looking brown and rusty you might wanna swish a couple of times and see if if helps. once you feel like the tank is clean put the petcock back in and fill it up for real. you can put regular old unleaded gas in there; the warning is to prevent people from putting kerosene and shit like that in the tank and expecting the bike to run. mix 2-stroke oil in at a 40:1 ratio (approximately 100 mL per gallon of gas) and you should be cool. might wanna pull the carb and give it a good dousing in carb cleaner, and put a new spark plug on it. you can get a little inline fuel filter if you feel that your tank is really sketchy; a little rust or some debris in the gas will foul things a lot less quickly that way
next you say you have a mk II sport which means it is two speed; the two speed gear box is a lot of fun and you want to make sure it treats you right. this means that you should put fresh tranny oil in the gear box. this is the big rectangular thingy on the right side of the bike that says PUCH on it. you can do this two ways.
the way that you really SHOULD do it is, remove the six flat headed screws that secure the transmission cover to the bike and pull the cover. there are a couple washers that might come out with the cover, so keep an eye out for them and the order in which they come. dont want to lose em. account for the washers and then tip the bike, draining out all the tranny fluid that is probably raunchy by now. keep an eye on those washers. once all the fluid is gone wipe up the dregs, put the washers back on the shafts if they happened to come off, and reattach the transmission cover. watch out for the bolts - there are two or three different lengths of bolt, each specific to one hole. make sure you put the right bolt back in the right hole. it is pretty obvious when you are screwing it back together which bolts correspond to which holes. don't force anything. also when you are seperating the transmission cover from the transmission you see there is a little oil paper gasket between the cover and the transmission; try not to mutilate or tear this in taking the cover off or putting it back on. once you have the cover on, grab a jug of TYPE F automatic transmission fluid. you see there is a little drain screw on the face of the transmission (the one smaller screw besides the six that you had to remove. it is around the P in PUCH approximately or so...) remove this screw. then if you look at the top of the transmission there is a little plug, remove this and use a funnel to fill the transmission until fluid starts to leak out of the drain hole when the bike is standing perfectly upright. once you're done, stick the plug and drain screw back in and the transmission should be solid (assuming it works to begin with).
if taking the transmission cover sounds intimidating to you, you could just use take out the drain screw and tip the bike, and try to drain the fluid as much as you can that way, and refill as above, but you will not get rid of a lot of the old dregs that way, so you dont get totally the effects of fresh tranny fluid.. but its probably better than nothing. either way, you should do something about the tranny fluid.
um, check tire pressure against the little metal plate on the bike. you should then be ready to fire it up.. if you have any further problems then something is wrong with the bike.