The >>>REALLY IMPORTANT<<<
thing is to not overtighten the plug when you reinstall it. See this
for what can happen. There are several such threads here. Use a new crush washer every time (they look like copper but are actually steel). A torque wrench is highly advisable, particularly if you're new to such things. There's not much to changing the oil but get a service manual and look in the preventive maintenance section.
I got fed up with the craziness and switched to an aluminum plug and an elastomeric seal. Now I tighten it just a bit beyond hand tight. But I also use safety wire to make sure that it doesn't back out. There's a thread here about it but the pics don't display any more.
Don't know what the service book is about but keep a log of what you do, when you do it, current mileage, and save your receipts.
P.S. You may also be interested in changing the oil filter. Suzuki sells the 09915-40620 wrench for that. But it's nearly $25. The earlier 09915-40611 wrench is still available, works fine, and is half the price or less. The only filter that I would use other than the Suzuki part is the Mobil-1 M1MC-133 and there may be availability issues in the UK. I try to prefill the filter before installing (you'll only be able to get it about half full because of the installation angle), clean the sealing surface on the crankcase, and smear some oil on the rubber seal. Pay attention that you don't cross thread it.