I would begin with an Arduino Nano, a breadboard, a 16x2 LCD display with i2c backpack and a bme280 sensor. This sensor can measure humidity and barometric pressure in addition to temperature and is as easy to use as simple temperature sensors.
This will get you started slowly, which is important if you are a beginner.
Once you have that working, get a second Nano and breadboard and a simple 433MHz transmitter & receiver pair. With these extra components, you can achieve a first prototype. But there will be more to be done after that.
The most difficult parts of the project, in my opinion, will be making the outdoor sensor last more than a few days on battery power, and protecting the outdoor sensor from the weather so that it will work correctly but not be damaged by heat, cold, rain, frost and so on.