You should start the game by selling the red tractor and, for a small amount of money, exchanging it for a model that has at least 140 horsepower - although it seems like a small change it is reflected by the pace at which some machines work. You should also think about buying a Pick-up truck - you can use it to transport the goods that you want to sell, to bring items that you bought in a shop and complete missions. Remember that the game is focused on modernizing your equipment so you should do it regularly - when you sell a vehicle or a machine you get 3/4 of its basic price.
Start with preparing your two fields. Rapeseed should be your first choice - it is much more profitable than wheat, although wheat allows you to get straw which can be used as animal bedding. But you shouldn't worry about this in the beginning of the game. You should deal with livestock later when you can buy 10-20 animals at once because smaller quantities are simply unprofitable. You should also sell supplies from your warehouse. Theoretically, you can wait for an event - they usually happen once a day and it can be a mission or a high demand for a given type of goods - you can get up to 2.0x more money. But in order to have a quick start you should sell all your supplies.
Now, you must buy a fertilizer vehicle - it is cheap, it costs 11 000$. However - one, completely full tank lasts for about one field and fertilizer, that is used to fill it, costs about 2 800$. Because of that you should try and get manure. Fertilizing increases your crops by about 30%.
Next, you must buy a lawn mower. You should know that grass that you mow is not wasted - a new one will grow from it and so on. If you don't want to pick it up now - it is okay. Later, you can collect a full semi-trailer on one green belt. Your first vehicle that can be used for collecting grass should be a forage wagon. You can also use it to collect straw from wheat. To get the most of it you need a powerful tractor. A mowed grass can be stored in livestock (cows and sheep) or you can take it to a biogas installation - there, you can get a few thousands for a full semi-trailer but what is the most important is that after you deliver grass you get a small percentage of slurry. After filling a silos you must wait some time until you can fill it again and get fertilizer (in the meantime you should store your harvests in other silos - there are three in total).
When you get enough money you can try to modernize your cultivation or planting vehicle (only one option in both categories). You should also buy a new field - for 100 000$ you can buy a field south west from yours. Two tractors and the default combine allows you to deal with three fields without any problems.
Now you can start with breeding livestock. In the beginning you should only have one type of animals which can be either cows (at least 15) or pigs (at least 25) - thanks to them you can get free fertilizer. If you want to have pigs you need to plant rapeseed, corn and wheat (for straws, wheat can be sold). If you want to invest in cows you need to buy a baler. This is the only way to produce hay (first, you must use side-delivery hay rake on a mowed grass). All animals have two requirements: pigs - rapeseed and corn; sheep and cows - grass and hay. If you won't provide any of the required elements for pigs their reproduction and ability to produce fertilizer will be reduced by a half. This doesn't concern cows/sheep - you only need to provide either hay or grass (depending on what you want to harvest).
In the later stage of the game you can buy new vehicles, especially another tractor (so you can quickly prepare your field for new crops by following a combine) and a new combine when you decide on having a new additional field. You must always remember to fulfill your animals' needs - when you have a lot of them they become very profitable but if they are something "additional" to you then you simply waste your time - if so then you should go back to crops so you can sell them later.