Congratulations on getting your ham license!
Mobile antenna installations are much less predictable than installations made for fixed locations. The mobile antenna installation is affected by the poor ground that the vehicle presents and the (typically) offset location of the antenna on the vehicle. As a result, most mobile antenna installations are compromises that require more trial and adjustment than a typical home antenna.
A good SWR meter or, even better, an antenna analyzer would be a good addition to your toolkit. It will receive a lot of use throughout your ham "career". If you cannot invest in one now, you probably could borrow one from a nearby ham or ham radio club. Sometimes an elmer (mentor) comes along free for the ride!
Start by adjusting your antennas per the manufacturer's instructions. This gets it into the ballpark. For most mobile antennas, you cannot directly calculate the length of the antenna as a function of wavelength because the manufacturer has added inductors or capacitors to the antenna that will throw off normal antenna calculations. Instead the manufacturer will include a chart or graph to help range in the adjustment.
From there you will need to use the SWR meter or antenna analyzer to adjust the antenna to the optimum setting. The advantage of using an antenna analyzer over an SWR meter is that it gives you much more information to help optimize your antenna system.
Once you get your antenna adjusted and you are ready to begin mobile operations, be prepared for the unexpected. Generally a mobile antenna is much less efficient than a home antenna. So at times you will be frustrated that you can hear stations that you cannot work. When I add the "mobile" suffix to my call, I have found that other stations are more likely to look past the more challenging conditions in order to make the QSO. On the other hand, I am often surprised to have a European station answer my mobile CQ - that's the magic of amateur radio. Enjoy!