From: Initium; a first Latin course on the direct method by Appleton & Jones
“Iulii Exitium”
Ita, clārissimē. Scīs populōs validōs vincere,
bella fēlīciter gerere potes, bellōrum pericula
nunc Rōmānī nōn timent; itaque benevolum
ergā amicōs tē praestā.
. . . therefore be kind towards the friends I ???
I’m stuck over tē praestā (imperative, present).
Apart from that not even sure which of the many meanings of praestare applies.
With se, to show, prove, or behave one’s self as…
Lewis, C. T., & Short, C. (1891). Harpers’ Latin Dictionary (p. 1431). New York; Oxford: Harper & Brothers; Clarendon Press.
So, “behave kindly towards…”
Got it now! Multas gratias tibi ago.
One strategy I’ve found for dealing with Latin words with multiple meanings is to try to guess the meaning from context and then see if I can find that meaning in the lexicon article. It also helps to determine how the word is being used (or how you think it’s being used). If it’s a verb, is it being used in an active or passive voice or as a deponent? Is it being used impersonally? Is it being used reflexively? Transitively or Intransitively? What mood is it in -indicative, subjunctive, infinitive, imperative? Knowing this information can help you narrow down your search. Same goes for nouns-knowing the case a noun is used in may yield a specific meaning or an idiomatic usage.
In the case of praesto, you guessed that in this context, it probably means “to be a certain way”, in other words, behave. If you look at the L&S article for praesto, the first thing you notice is that it can be used as an adverb or as a verb, so skip down to the section dealing with the verb. Now look at the possible meanings, which are highlighted by being in italics. Remember you’re looking for its reflexive use:
Section I has meanings associated with “stand before”, so the meaning you’re looking for probably isn’t there.
Section II has three sections A,B, and C listing meanings in the active voice:
A. Surpass - probably not
B. To become surety for - not really
C. (gen.=general meaning) To fulfill, discharge, perform (possibly). Well, this is closer to your meaning than the other two, so let’s look at particular meanings (Section C2)
a. keep, preserve, maintain (close!)
b. show, exhibit, prove, manifest - this is worth exploring further - ah! “With se, to show, prove, or behave one’s self as”
Guess confirmed.
Pianophile, if I’m “preaching to the choir”, forgive me. For that matter, there others on this board far more learned than I am who may have a better system. This is what works for me, so I’m passing it along to help you and others make more efficient use of lexica and dictionaries. I’ll admit, Lewis & Short was pretty daunting when I first started to use it. It makes looking up German Sprichwörter seem dead easy! As an additional aid, here’s a link to the abbreviations at the front of the dictionary:
https://archive.org/details/in.ernet.dli.2015.147309/page/n13/mode/2up
Aetos - Many thanks. Please don’t stop preaching to as many choirs as you like - my big mistake was - and seneca will love this! - I tried to fit the English I had in mind ". . . the friends I line up before you/“stand before” you) to the Latin - but then came a cropper over the reflexive imperative. And once fixated on “stand before” I just couldn’t extricate myself from that trap.
I dont know this book so I looked at it on archive.org. I see that the vocabulary is partly ( mainly?) given in Latin although some words are glossed in English. I wonder whether this is a suitable book to tackle without a teacher? I see from the preface that "This book is intended for Latin beginners who have spent a fortnight on purely oral work without seeing any text-book. How the preliminary lessons are conducted will be found in the “Teachers Companion to Initium” published along with this volume. " Have you a copy of the Teachers Companion?
I think that the “direct method” doesn’t involve any translation nor looking up words in a Latin English lexicon. Doing so rather defeats the purpose of that method. I dont mean to be discouraging but I wonder whether this book is the most helpful for you, especially without a teacher familiar with how the book works?
Edit: please feel free to ignore this post. We are all different and learn in different ways. If it suits you - carry on and people here will try to help.
I am not carrying on with Initium - but just felt I needed a few days’ break from Familia Romana Cap XX which has a lot of new things and I was feeling a bit low, wondering if I could cope. And I’m certainly not ignoring any of your posts! Every bit of help and advice is welcome and sorely needed. 
I know the feeling! Learning on your own can be a daunting process. We all have different strategies for dealing with those lows when we doubt ourselves. As well as looking at other textbooks you could try re-reading the beginning of Familia Romana to remind yourself how far you have come.
Aetos has given you excellent advice. I would only add that sometimes its worth spending a bit of time trying to see how the meanings of say praesto are related. At first sight they look very different. Remember that meanings given in dictionaries are derived from the context in which a word is used and so they may be used by authors in quite different ways especially over time. I think that’s why for learners a context specific vocabulary is very helpful.
Its also worth looking at this https://geoffreysteadman.com/5-lines-per-day/. As Steadman says habits do matter!
Good luck and dont be shy about sharing how you feel about how your studies are going.
It was this post by Shenoute addressed to someone else which made me have a look at “Initium” thinking it would be easy-peasy - but then was in for a shock.
Your Steadman link is full of good advice - most of which I already follow, except I don’t have a group to share with.
I will go back and re-read Familia Romana - have been hesitating in case I’d find I’d forgotten most of it. Well, we’ll see. 
Share your progress here with us!
I have an epub of Puer Romanus on my site that may be better than the version available on the internet archive;
https://www.moleboroughcollege.org/latinlibrary