How do I put 10101011 from binary into decimal?

As with all binary numbers, you can start from the right side. The furthest right place represents the 2⁰ (=1) place. As you move left, the places represent 2¹, 2², 2³, etc. So, as you go from the right to the left, you can find the decimal representation by multiplying the binary digit by the place it is in. Then, you should add up all of these numbers.
Using 10101011 starting from the furthest right, 12⁰​ = 1, 12¹ = 2, 02² = 0, 12³​ = 8, 02⁴ = 0, 12⁵ = 32, 02⁶ = 0, 12⁷ = 128.
Now, add these numbers together: 1+2+0+8+0+32+0+128 = 171.