So I'm stuck with this formal proof problem. This is what I have so far. If it's unclear what I written below here's the link for a more clearer picture: https://i.stack.imgur.com/TAZ7u.jpg
I'm really not sure if what I did is correct so far, I think I may be missing some things. I just need guidance in how to reach the goal. Please help me. Thank you so much in advance!
Given that your goal is $Medium(b) \land Dodec(b)$, you should definitely not assume that! So, get rid of that whole outside subproof.
Also, you should make it a habit to, whenever you start a subproof, to figure out what the last line of the subproof should be, and what you are going to do with the subproof after you close it, all before you actually start making further steps inside the subproof. That way, your proof, as well as your thinking itself, keeps much more organized. In fact, had you done this, you would have realized that assuming what you try to prove is never goig to get you anywhere.
Finally, you should look at your premises and your goal and come up with an infomal proof or strategy first before you start makig inferences, otherwise you risk just spinnin your wheels. In this case, if you just think about the premises, and what they mean, it should be pretty straightforward how to get the goal from that: combine the $\neg Small(b)$ from premise 3 with premise 2 to get $Medium(b)$, and combine the $\neg Cube(b)$ from premise 3 with premise 1 to get $Dodec(b)$ and then jus conjunct those two statements to get your goal.
In both cases, you have to follow the Disjunctive Syllogism pattern: this is not a direct rule in Fitch, but the Disjunctive Syllogism pattern of $P \lor Q$, $\neg P$, therefore $Q$ should be in the book, and something you should memorize if you want to get good at these proofs, since this pattern comes up over and over. Indeed, you do this twice in your proof, so here is the general pattern:
$P \lor Q$
$\neg P$
$\qquad P$ Assumption
$\qquad \bot$ $\bot$ Intro 2,3
$\qquad Q$ $\bot$ Elim 4 (end of first subproof)
$\qquad Q$ Assumption (start and end of second subproof)
$Q$ $\quad \lor$ Elim 1,3-5,6-6