You need a jeweller's screw driver set as the screws have a special set angle , these screw drivers are close enough to the right ones, or a set can be purchased from an electronics store. You have to take note of the order and most importantly the position of the barrel screw position in relation to the rest of the lens assembly. Once you have removed all the accessable screws ( you may also find you have to prise out the plastic ferrel from the front of the lens to expose the screws). Now you can take apart the lens and unscrew the barrel ( may be left or right hand thread. Once apart use opticians cleaning fluid, available from all eyeglass suppliers. In most cases this will be sufficient to clean fungus, if not you will have to resort to powerful solutions of hydrogen peroxide and ammonia. Which is in my opinion is when you should probably get it professionally cleaned.
Regards Ken.