Initially you have to create your own developer account to experience the Salesoforce magics. Please click here to  create your developer account.If you already have your account then proceed with below steps.

Follow the below steps to Build a Custom Object

1. Go to your developer account Salesforce page
2. Click the Setup icon in the top-right navigation menu and select Setup.
3. Click the Object Manager tab next to Home.
4. Click the Create dropdown and then select Custom Object.
5. In the New Custom Object form fill in:
   1. In the Custom Object Information section:
      1. Label: NewObject
      2. Plural Label: NewObjects
      3. Object Name: NewObject
   2. In the Enter the Record Name Label and Format section:
      1. Record Name: Reading Id (replace the default NewObject Name).
      2. Data Type: Auto Number
      3. Display Format: R-{0000}
      4. Starting Number: 1
   3. Click Save.
6. On the NewObject Custom Object page, make sure that the API Name is NewObject__c.
7. Select the Fields & Relationships section from the left navigation.
   1. Click New.
   2. Create a field for the temperature.
      1. In the Step 1. Choose the field type section:(We have many field types like Number,Text,Date and etc.,)
         1. Data Type: Number
         2. Click Next
      2. In the Step 2. Enter the field details section:
         1. Field Label: temperature
         2. Length: 4
         3. Decimal Places: 2
         4. Field Name: temperature.
      3. Click Next, Next, and then Save & New.

By Enabling the Check box of “Always require a value in this field in order to save a record” you can make the field as a required one.

By Enabling the Check box of “Set this field as the unique record identifier from an external system.” you can make this field as a unique record identifier from an external system.

By Enabling the Check box of ” Do not allow duplicate values” you can avoid duplicates.

Also you can set up the default value to the fields.

This training blog will be very helpful to learn about object and field creation in Salesforce.