The principle samaya is to have respect for the vajramasters that have bestowed empowerment upon one. Also vital is training in bodhicitta. Then, in general, the distilled essence of keeping all one's samayas is to train in the view of all appearances, sound, and thoughts as the three vajras. Now, you may not be familiar with what the three vajras means, and you are not yet very knowledgeable about how to practice Vajrayana... Well, the teachings say that we cannot break vows which we don't yet understand or have the capacity to accomplish, and we all started just where you now are. If you were to look at a customary presentation of the highest yoga tantra samayas, you'd see that there are samayas to accomplish all kinds of things we can only now aspire to. So what do we do? We make aspirational prayers, we try to create the causes to study with a competent, qualified lama, and we train in what we're able to: making sure to always maintain respect for the lamas who gave us empowerment, training to always have compassion for sentient beings and the wish to help them all attain liberation, and training our way toward equanimity toward close ones and not-so-close ones. We train in bodhisattva-like conduct, such as the 6 paramitas. Most important to understand is that the principle of samaya is not that one is supposed to become perfect the instant one receives empowerment and takes vows. It is a training. Generally speaking, you only damage the samayas in proportion to your capacity to uphold them. Also, generally speaking, there are at least 4 parts to a fully broken vow, including samaya, and one generally has to pretty deliberately break most of them. Anyway, again, what we do is train in what we know how to train in now, and make aspirations and attempts to create the causes to be able to learn further from a qualified lama in the future.
Now, you've been given the lung for a ngondro. A perfectly fine way to guard and purify your vows would be to do that daily. Most ngondros are very straightforward and would be hard to make important mistakes with. Plus, there are translations of commentaries on many of the most popular ngondro liturgies. Receiving the lung for a ngondro necessarily means a lung for the Vajrasattva practice within it, so that would be an excellent practice. It is said that reciting at least 21 of the long Vajrasattva mantra at the end of the day (with single-pointed concentration and the four powers) is powerful enough to purify whatever damages or breakages to one's samaya that day. Also, as soon as you catch yourself doing/thinking/saying something unwholesome, the sooner you confess it and do the recitation, the easier it is to purify, so you can go somewhere private and quietly, briefly recite a few Vajrasattvas as soon after as you're able, and then do a thorough Vajrasattva practice (or the whole ngondro, depending on your desire and time) before bed. So, no need to worry.
And yes, all lamas who bestow empowerment on you are vajramasters, but this does not mean you are obligated to become their disciple. You consider them as an extremely kind, noble teacher to you (because even if they just bestow the empowerment upon you and little else, the seeds for complete realization are indelibly planted in your mindstream to eventually blossom) and try to view them purely and respectfully. But eventually, when you come to feel that a particular lama (or lamas) has really been fundamental to you gaining some personal experience and knowledge of your true nature, that will be your root lama and you'll have no doubt. Usually at some time before that, through our experiences of receiving teachings and guidance from one or more lamas over some time, we come to particularly appreciate and trust a certain lama who's way of teaching and apparent knowledge and wisdom resonates with us and upon who we principally rely.