Long not-quite-an-answer, actually.
First of all, in general in Italian you count the syllables from the end, rather than from the beginning. So, most words are piane, that is, the (main) stress is to the last-but-one syllable. The words casa, lavoro, panettone all belong to the same category, even though the stressed syllables are, respectively, the first, the second, and the third one.
Secondly, all your examples are of verbs with a clitic pronoun attached at the end, which muddles things a little. The good news is that the stress in the verb itself doesn't move: làva > làvati; pulìsci > pulìsciti and so on. (Accents are not usually written down, but I'm writing them here for clarity.)
So we are left with just the “pure” form of the imperative mood: lava, pulisci etc. (All the examples in the question are in the 2nd (singular) person, but the imperative mood also has a 5th (or 2nd plural) person: lavate, pulite and so on.)
In modern Italian, in most cases, these forms are equal to the 3rd person (for the verbs in -are) or the 2nd person (for the verbs in -ere or in -ire) the in the present of the indicative mood:
(lui) lava = he washes; lava! = wash!
(tu) tieni = you hold; tieni! = hold!
(tu) pulisci = you clean; pulisci! = clean!
So the problem reduces to learning how to conjugate in the present tense of the indicative mood a given verb, and this is mostly regular (according to the different classes of verbs) and is anyway easy to find for specific verbs in any dictionary.
