You can put the heater on now to bring the tank up to temperature - should be around 26 to 28 for most setups.
OK some further advice:
This will be a bit of a read, but as I guess your a 'bona fide' newbie, it may help to give you a good all-round start to keeping our slippery friends. So read on...
Canister filter maintenance:
You will need to clean your external canister filter about once a month - don't listen to advice that tells you that you only do this every two to three months as the filter and media will become heavily clogged up and will be much more difficult to clean and may even result in you having to replace all the internal media as you won't be able to clean it sufficiently! Don't be put off by the below maintenance routine as they are by far the best choice of tank filtration for most setups.
1. Switch it off - the canister filter
not you!
2. Uncouple the tubes as per the instructions.
3. Unclip the power head from the canister and remove the canister body - carefully as it is still full of water - and put a bowl under the power head as it will still contain some water and will make a mess.
4. Drain some of the water out of the tank (about 10% of the tank water) into a bucket / container that you don't use around the house - cleaning chemicals would 'leach' into the water and poison your fish - even if it is perfectly dry!
5. Empty the canister water into the sink / water your plants with this as they would love all the stuff in there.
6. Take the filter media out of the canister and clean it in the water that you just removed from the tank - this way you won't kill all the good bacteria built up in it - if you clean the media under the tap you
will kill
all of the bacteria.
Clean the filter sponges by holding them in the water and giving them a good squeeze until them appear
fairly clean - you don't want to remove all of the bacteria remember.
7. Put all the media back in the canister - but leave it empty of water.
8. Take the bucket / container and place it under the filter tubes that go into your tank. Turn the lever so that the water in the tubes drains into the container - should only take a few seconds to happen. You do this so that you avoid all the gunk in the inlet tube from being pumped straight into your nice clean tank when the pump is restarted.
9. Put the canister back in position, clip the power head onto the canister body and reconnect the tubes correctly. Then turn the lever so that the water from your tank drains into the canister and fills it up - should take one to three minutes to fill completely.
10. Switch the filter back on and wait to see if the water starts to flow into you tank correctly - if not then give the 'primer' a few pumps to get it going (If you have to do this it may pay to do this without the white ball in place, but make sure you put it back - while the filters still running.) You may still get a bit of noise from the canister at first, but, if so, follow Water Drop's advice and dance round the living room with it until any air bubbles have gone.
Sounds like a lot of hassle to go through once a month but after you have done it two or three times you should be able to get this job down to about fifteen minutes - start to finish! Remember, to maintain your tank well, and keep the fishes happy, no job you do should be regarded as a chore!
I don't mean to sound patronising, but I run Fluval externals myself (two 405s) and they are not the easiest to maintain and this advice comes from my personal experience / pain with the darn things.
(I have since replaced one of my Fluvals with an AquaPro 3 and it is far better and easier to maintain - and much cheaper to buy in the first place!)
Water changes:
Once a week (or as part of the canister filter maintenance) you need to change around 15% of the tank's water. You do this to reduce the bioload on the tank, keeping things like PH, nitrites and ammonia levels acceptable to your little fishes and to keep the water nice and clear. You should also clean the inside of the glass (front and sides - don't worry too much about the back) so that you do all the disturbance to the tank in one go and it makes you feel better as the tank will now look super clean!
When you replace the water, use the bucket / container that you don't use around the house (see canister maintenance point 4 above) and fill it with enough water to replace what you have removed, adding the correct amount of dechlorinator - use something like StressCoat as this will also give some added benefits to the fish. When you do this don't be afraid to use some hot water from the tap or add some hot water from the kettle - enough to bring the temperature of the water in the bucket to about the same as what's in the tank. This way you won't shock your fishes with cold water and also your tank heater won't have to struggle for an extra hour or two to bring the tank back up to its correct temperature.
Hope this helps
have fun 
