Thirst for water, cravings for sweets...not sure those are exactly early pregnancy symptoms. Each person is different, but very early in a pregnancy, a woman may notice that her breasts are EXTREMELY tender, she always has to urinate, and she is tired all the time. Then, she will miss her period or have a spotty one that is very light. By then, her stomach is feeling bloated and hardened (if she isn't carrying much extra weight). This may prompt a woman to take a pregnancy test if she hasn't gotten a period by then.
Some women know from the get go if they are pregnant, and others don't know until the pregnancy is quite advanced. (I call this denial). Pregnancy tests are not 100% reliable, especially if they are not taken properly. They are best used with first morning's urine (which is most highly concentrated with pregnancy hormones), and best taken when a woman has missed her period at least 3 days to one week. (Some jump the gun and take it too early).
There are free clinics and Planned Parenthood locations in most larger cities and towns. They offer free or reduced fee pregnancy tests for any woman needing one. Run a Google search for where to find one in your area.