Marx and Engels wrote a great deal, on a wide variety of subjects and over a long period of time. Some of their writing was in response to political issues of the day which are long forgotten, some was concerned to criticise opponents who held views now rarely encountered, while some was of a very abstract and philosophical nature. So it can be very difficult for someone with no previous acquaintance with their work to know where to begin. And diving in at some unsuitable place (Capital, vol. 1, ch. 1, for instance) may discourage further exploration. Their writings on economics, on history and on politics were for Marx and Engels closely interrelated.
Under capitalism, wealth takes the form of an immense accumulation of commodities, so Marx begins Capital by an analysis of the commodity, which leads to the distinction of use-value, value and exchange-value. The value of a commodity is the amount of labour-time socially necessary for its production. The air has use-value but no value, as its usefulness does not result from labour. Different forms of value are discussed before Marx turns to the fetishism of commodities, whereby commodities seem to take on a life of their own, rather than being seen as products of human labour. Then comes a discussion of money, the universal measure of value and means of exchange. A general rise in the price of commodities may result from a fall in the value of money (inflation). The exchange of commodities follows the circuit Commodity-Money-Commodity (C-M-C). The total of such circuits is the circulation of commodities and the starting-point of capital. Alongside the C-M-C circuit is that of M-C-M (i.e. buying in order to sell), and money which circulates in this way is potentially capital. Money which begets money (M-C-M’) is the general formula of capital.