Remove the cover on the side (big wheel) and take off the chain. Use a stiff brush to remove any oily debris.
Count the links - put clothes peg on one link as a guide - buy replacement chain (£17) with same number of links and put it in - adjust tension - I set mine to have 3mm play when cold.
When you have the bar out remove sawdust from the oil holes - you are also encouraged to reverse the bar each time you change the chain to even out wear.
A DIY store will sell you a chain sharpening kit for about £10 - takes about 20 minutes - relatively easy. Good sharpening article here...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix circular saws.