Bismillah,
Asalamu Alaykom,
Alhamdulilah you have read about the incident of Ghadir and May Allah swt guide you to accept the divine authority of Imam Ali (as).
Firstly after becoming Shia, you will need to refer to the jurists who specialise in ahkaam or the Islamic laws. You will find some of the experts who say it is allowed to pray behind a Sunni imam provided that you recite the surahs yourself; others will say you can only do so out of taqiya (when one fears for their life & must conceal their faith).
Then you’ll find scholars who will say that one cannot pray behind a Sunni imam at all due to justice being one of the conditions as well as it being crucial that one prays correctly. Unfortunately our brothers who refer to themselves as Sunnis, perform both their salah and wudu in the incorrect way.
So when you choose a marja then you may check their ruling.
About the Sahaba, one can only respect the Sahaba who were PROVEN in history to be loyal to both the prophet Mohammad & his Ahlul Bayt (peace be upon them all). In other cases we may assume the best of those Sahaba who have nothing bad reported about them in history.
It has been proven from the Quran and hadiths of both sects, that there existed many hypocrites among the Sahaba. In the authentic sources of the sect who refers to itself as ahlus sunnah, you’ll find the prophet Mohamamd (sawa) clearly saying that among his companions were hypocrites therefore, how can one respect all Sahaba?
For example, can one love Muawiyah who encouraged people to curse & insult Imam Ali?! Of course not! Therefore one would have to disassociate from such people despite them being 'companions'.
Can one love Umar who threatened to burn down the house of Fatima Al-Zahra (as) and imam Ali (as)?
As for those companions who are proven praise worthy, then you may show your love towards them.
If you research the above matters you will find various sources which mention the points I have summarised.
May Allah grant you success
