Let's look at the position and try to evaluate the situation on the board. This is always a good idea, especially if you're struggling to come up with a concrete plan.
r1b2rk1/4n3/p1p1p1p1/1p1pPp1p/1PnN1P2/2NB4/2PP2PP/1RB2RK1 b - - 0 1
HOW TO EVALUATE THIS POSITION?
In this position black is a piece down for a pawn, so this in and of itself should be cause for concern. But this is actually not the only issue with black's position, although the material situation is of course the easiest thing to consider in a position since it doesn't require anything besides basic arithmetics to calculate.
Factors that are just as important, but often much harder to appreciate for beginners, include king safety, piece activity and coordination, pawn structure, space, etc. Let's begin by looking at black's pawn structure, as it will make clear just how problematic black's position really is.
The "sawtooth" kind of structure that black has set up is something I often see beginners play for, as they feel like it's easy to make sure that everything's protected. But the structure has some serious flaws. First off, the pawns in the back of the structure (g6, e6, c6, a6) are at serious risk of becoming weak since there is no way to defend them with other pawns and it's very hard to advance them without losing them; for instance, moving the g- or c-pawn forward here just lets white capture the pawns for free. But this is not where the problems end. The pawns that are further advanced (h5, f5, d5, b5) are also virtually immobilized; if they were to move forward they would immediately lose support and be vulnerable to attack (imagine if black were to play ...h4, then white would win that pawn if they just manage to threaten it somehow). So in the end, black's pawn structure is rigid and inflexible, and much weaker than it seems at first glance. Another problem with this structure is that it leaves your dark squares very weak. If you want to defend a dark square it has to be done with a piece, and since you're lacking a dark-squared bishop it will be difficult to do this efficiently.
The inflexible pawn structure causes a lot of other headaches for black as well. For instance, black already controls less space than white (in this case it can be clearly seen by the fact that black is relegated to play on the 6th to 8th rank with almost all of their pieces, while white's pieces have a lot more opportunity for movement). This means that black will have less space available to move around their pieces. This, in turn, makes it harder for black's pieces to coordinate without stepping on each others' toes which makes it harder for black to react to white's threats in the long term.
So, to summarize, black is material down and has a weak and inflexible pawn structure which makes their position cramped and gives their pieces a harder time to move effectively (the bishop on c8 is especially terrible here). The only "good" thing about black's position is that their king is pretty safe at the moment. Therefore your aim in this position as black is to draw, since winning is out of the question as long as white doesn't blunder something serious.
HOW DOES WHITE WIN?
Depending on the opposition a draw for black can be easy or difficult or impossible to achieve. A grandmaster would easily win this position as white no matter what black does, the average player could potentially struggle against optimal play and a beginner is probably not going to win this as long as black hunkers down and plays passive defense. Let's discuss a bit how white actually wins this position so that we know what we must try to prevent.
At the moment, the position is pretty locked, so white should ultimately aim to open it up somehow. An open file is enough here to work with, so a pawn breakthrough with h3-g4 is going to be enough to open the position eventually. But the fact of the matter is that with a locked central structure there is no real need to play this immediately, and white can instead focus on improving their position by developing the dark-squared bishop on c1; if this bishop were to awaken it could easily become very strong, since black's dark squares are desperately weak as I described earlier. In fact, a pawn break with h3-g4 may not even be necessary to win here if the c1 bishop manages to worm its way into black's position.
In general, white should try to use their extra space to their advantage in order to "optimize" the position of their pieces and threaten as many of black's weak points as possible at once while at the same time take away squares for black's pieces to occupy safely. Therefore, the following would be an excellent sequence of moves for white to be able to play: Nb3 (this opens the potential scope of the bishop on c1 later on, and it blocks black from making any pawn breaks; also, this knight could be very annoying for black if it were to reach c5, as this would practically immobilize the bishop on c8 completely), Be2 (opens up the d-pawn, and it also starts to prepare for a future g4 pawn break), d3 (pushes black's knight away from its active post on c4), Be3 (this bishop may go c5-d6 or f2-h4, depending on what black decides to defend against). If white so pleases, they could begin the entire operation by playing Be2, d3, Be3 first.
SO WHAT SHOULD BLACK DO HERE?
This is by far the hardest question to answer. Black is objectively lost here, and to be honest, I'm struggling to see a decent way for black to play this in order to make life hard for white. If I were playing a weaker opponent I would probably just hunker down here and play passively, hoping that white doesn't find a winning plan (which they probably wouldn't find, at least immediately).
However, the problem with this is that white just wins then by playing along the plan I outlined above, and this is something I kind of expect the average player (~1600 Elo) to find eventually. So I guess that ...a5 is really the best move to try here in practical terms. This move is good to try because it eliminates the pawn on b4, which gives black some chances to take back some terrain on the queenside by means of just rolling their pawns forward if white is careless enough. This would solve the space issue nicely, and give black's pieces a fighting chance to activate themselves. Note, however, that black is dead lost with decent play from white here and even if ...a5 is relatively best it's still far from nearly enough to ensure a draw. For instance, the line 1...a5 2.bxa5 Rxa5 (if 2...Nxa5 white just plays Ba3, activating their dark-squared bishop immediately and threatening the knight on e7) 3.Nb3, and white dominates completely. But since the pawn on b4 has been removed this means that the pawns on the queenside are a bit more mobile, which means that white has to be a bit more tactically alert.
