Hey mate - welcome to the forum.
Not sure if you are after advice on what to wear on wet days or how to ride on them (or something else alltogether) so I'll list a few things for you!
Gloves: always good on wet days to have a couple of pairs on hand, particularly being a new rider as your'e gloves wont just get wet from the rain etc but also from manhandling/picking up your bike etc. Some people like to wear rubber or latex gloves inside their bike gloves to help with cold hands (stopping wind chill etc) but I've never personally gone down this path. A tip for your gloves is when ever you have a break take them off and lay them on your exhaust to warm them up and dry them out a bit (obviously keep an eye on them doing this to make sure you dont melt them etc!)
Jackets: I have tried a couple different spray jackets (including ventilated riding specific Fox ones) and generally dont bother. I find they are only beneficial for stopping wind chill, they may keep you dry from the rain but you sweat so much that you end up just as wet inside them anyway! (note - thats my personal opinion, others will have other views). I find that with my body armour and flight vest mys chest is pretty well covered so its only really my arms that suffer from wind chill. also helps that the riding I do doesnt include much higher speed stuff as well.
Goggles: MAKE SURE YOU WEAR THEM! (again this is my personal opinion, others will have different views). unfortunately goggles tend to fog up badly on wet cold days which see's lots of people ditch them. some will carry a pair of clear safety glasses to wear instead (again, I havent tried this). I personally only wear goggles with dual layer lenses and rarely have any issues with fogging. (if your bike has a thermo fan that is switch operated, when your goggles fog up switch your fan on and hold them behind it, they will clear up in no time). for really really wet days I have a pair of old goggles with which I have removed the top and bottom dust foam which greatly increases the air flow.. obviously you cant wear these on a dry day as you will then get loads of dust in your eyes!
Feet: obviously the best thing to do is avoid putting your feet down in puddles or creeks etc, but unfortunately that is often easier said then done! I know some members have "waterproof" socks which you buy at camping and hiking socks and they swear by them (one day I may lash out and buy a pair!). I wear over the boot pants and find they help (reduce not stop) water from getting in from splash type scenarios (wont help when putting your feet down in knee high water! My last OTB pants had an inner gator (like a ski pant has) which were slightly better than my current ones. some say that the better your boots the better they are at keeping water out, but I wouldnt say my current Gearne SG12's are any better than my previous Alpine Star Tech 8's!
Riding: good question.... firstly carry plenty of "goggle rags" (the best thing ive found over the years are old face washers - I cut them into strips (3 strips from each face washer)) and dont be afraid to stop if your visibility gets too bad to either wipe them off or wait for the rain to back off - remember, SAFETY FIRST!
Depending on where you are riding conditions can change dramatically with only the lightest of sprinkles - a prime example of this is Kinglake (or anywhere with red clay), it can literally go from decent grip to ice skating with the lightest of showers.
When it gets slippery its often a good idea to ride up a gear (i.e. 3rd in stead of 2nd) as this will take reduce the potential for the back wheel wanting to overtake the front wheel when a little throttle is applied!
Be wary of puddles/bogholes (allthough this applies in the dry as well), particularly in well ridden areas such as Neerim (where the ruts have their own ruts!). They will often be deeper than anticipated and will often contain multiple ruts making easy to cross rut and sending you flying towards a splashdown!
Make sure you have good tyres. Ideally you will be running UHD (Ultra Heavy Duty) tubes. It can be helpful to lower your tyre pressures which will spread the tyres "footprint" providing better traction (especially with climbs). As a rule of thumb only, IF running UHD a good starting point is 12psi front and 10psi rear, in wet slippery conditions dropping the rear to 7 or 8 can be helpful but dont go much lower or you run the risk of pinch flats as well as dinting your rims on square edge hits. (again, I stress, those pressures are opinion only, its what I ran when using UHD tubes - opinions will vary and several variables can come into play such as your weight and the tyres you have etc). Make sure (like always) that you have a spare tube (just carry one 21" front tube - these can be used in both front and rear tyres on the trails!) and a hand pump - or air canisters. the pump can be handy if struggling on a hill, let the pressure down which will help you get up and then put some air back in before continuing on etc.
I guess the other thing is to make sure you have a towell and full dry change of clothes for the drive home!