I feel that the Fixed Income is really complicated, and I am having hard time to comprehend it. Can anyone give me some advise regarding how to study it?
What part?
Pretty broad question.
Yields up, prices down. Done 
Do practice questions until it makes sense.
Thanks!
So far I am in the Term Structure reading, I am trying to comprehend the material but it is taking so long and I feel that I do not fully comprehend it.
For example, I do not understand the difference between the spot rate and the normal discount rate that we use in the DCF; why does the spot rate change every year while the discount rate does not.
Why would normally the future rates have to be lower than the spot rates, is it because the spot price has to be higher than the future price?
These are the two main questions I have till I am done with the reading.
Well, they should both change; for example in year 1 to value your bond we would be using the spot rate and for a FCFF DCF, we would be using the WACC for that particular year. In year 2 (spot rate changed, your WACC most probably also changed), you are going to discount those cash flows with different discount rates as you did in year 1 (different spot rate, and different WACC). That is why
I don’t understand your second question.
Ok maybe I didn’t understand you so well. When you discount each bond’s cash flows with a different discount rate (in this case it will be the spot rate) you are are using the arbitrage-free approach for bond valuation.
You discount all of the free cash flows (to equity/to the firm) in a DCF by the cost of equity/WACC and not each individual cash flow by a different discount rate, because we are assuming that the discount rate represents the risk of each of those cash flows; however, if we thought that (assume 5 estimated cash flows) cash flows from year 1 to 3 will be riskier than those of 4 to 5, you should adjust your WACC to those specific risks of those cash flows, so you will be using 2 discount rates.