Now there is 2 ways to bid a house.
First by the website:
1. Login to your account on tibia website;
2. Go to the tab community -> houses;
3. Select your world/city and click Submit
4. Find interesting house, view on it and bid:)
Before you make a bid you need to be offline in game
Second way - in game:
1. Go to cyclopedia map:

2. Go to Houses:

There you can find all houses. You can sort all houses by size/name/price/auctioned/rented. When you wanna bid a house just click BID (I wasn't able to do that because i have bid on another character) and write you price.

After you bid you can see what is the current price, what is your limit and who is the highest bidder (I deleted my name).

After auction ends you get the house with the server save :)