You can sow grass using the basic sowing machine: choose the right seeds and start the process. You only have to do this once, as the grass will grown back by itself after some time. At the beginning of the game, you don't have to waste the area on your field to sow grass - you can mow it on nearby meadows and along roads. This way, you will get enough to for example start a sheep husbandry. To start getting grass, you need at least two machines - a mower (the cheapest one costs 8300$, but it is much better to save some more and buy FC 3525 for 13 500$) and a tipper (Euroboss 330T for 27 000$ will do for the start).
A good way to mow grass is to attach a mower at the front and a trailer at the back. This way, you will automatically pick up the grass you mow. Later in the game, when you will need a lot of grass (because of, for example, a large sheep or cow farm), it will be good to invest in the best mower available (it costs 52 000$) and attach it at the back of the tractor. This way you will mow the area on the sides and in front of the tractor. A new trailer is also a good idea. The grass can be sold in a shop located on your farm - you will get about 100$ for a ton. When you ha nothing better to do, you can earn some money this way, also completing missions connected to mowing.
Hay is dried grass. To "produce" it, you will have to buy a windrower (GF 6502 for 10 300$). This way, in the place where you drive with the windrower attached, the grass will dry immediately. It is a good idea to attach a mower at the front and a windrower at the back of the tractor. Next, you can load the hay onto a trailer and - similarly to grass - sell it in the shop. However, it is not very profitable, as the price is the same and you only have to work more. Hay will be more useful in the form of bales.
Straw is the byproduct of harvesting two kinds of grain. To be able to collect straw, you need either wheat or barley. Next, when starting the harvest, remember to switch on the appropriate option in the tractor that will allow you to get straw [,]. By doing so, you will leave straw that is ready to be collected, in places where you harvest. You can of course collect it using a proper trailer. For a ton of straw, you will get 80$ in the shop. However, it will be more useful for taking care for your cows, which will start to produce manure in return.
Selling it in the shop is not the only thing that you can do with grass, hay or straw. These products will be used for feeding animals. Sheep and cattle can be fed with normal grass, hay and straw is used to produce fodder for cows. To do that, you will need bales and to make them, you will of course need either hay or straw, left on the field. Then, using a tractor with a baler attached to it, you simply have to drive around the field, collecting hay or straw. When there's enough of it in the machine, the baler will produce a bale.
It will be necessary to buy a few machines - one of the bale makers (costs about 90 000$), a front loader and a special tractor (the cheapest version costs 64 000$). With this equipment, you will be able to make bales and transport them anywhere, to sell them or use for making fodder. Collecting larger amounts is much easier with proper machines. You can also buy a trailer and load the bales using a front loader.
No matter of what of which option you choose, you need to spend some money in order to be able to make bales (the price of the bale maker is the main factor here). However, bales will be necessary later in the game, to, for example, increase the productivity of your cow husbandry to maximum. Prices in the shop: a bale of hay (green) - about 840$, a bale of straw (yellow) - about 500$.