Welcome!!!
What you need to do is first decide what you want to do with the current decor and how you want to incorporate it into the new tank. If you want to put everything in the new tank then thats great because all the good bacteria is already attached to the current decor so you would be transfering it from the old to new with no problems as long as you keep it wet.
Once youve figured that out then you have to figure out if the filter you have now can support the new tank size. If not then you will need to get the new filter up and running on the new tank along with the old filter so you will have the good bacteria from the old filter seeding the new filter material. Again, do not let any of the seeded filter dry. Do not only hook up the new filter and put the fish in without the old filter, and depending on how big the new tank is and how much the old filter can hold you may see a mini cycle.
Before you move anything unhook the filter/heater/airstone etc.. Empty some tank water into a bucket and put the decor in there to keep it wet. Empty more tank water into another bucket (or the same if you dont have two) and catch all the fish and put em in the bucket. You now have about an hour to do your change so be quick.
Start moving the substrate into the new tank and scaping it the way you'd like. If you have new substrate try to have as much of the older stuff touching the water as possible so the bacteria can still help the new cycle along. Once you have done this begin filling and decorating how you like. Put both filters on there and run them side by side for as long as you can. Ideal time is a month, but unless you have a glass top its hard to have the top on and both filters, so you may have to go without a top for a while. Minimum time should be a week IMO.
Put the fish in the tank!! I skipped a few steps like adding dechlor, making sure water temp is right, rinsing new filter media etc.. so make sure you do that haha and finally...post some pics!!!!
If i forgot anything there will be plenty of good advice to fill in the holes