Hi egor and welcome to HT!
To start off why not look at our resort guides here
When you have picked a resort we have a a hotel review page here with loads of reviews!
I'm sure members will pitch in with their recommendations as well.

look at st konstintine/st elina the oldest resorts in bulgaria

we went to pomorie last year and booked again on return
if you want quiet/laid back this is the place
relax on the beach during the day then plenty of bars etc on a night
and it has two advantages over golden sands/sunny beach these are
young people have not discovered it
and its cheaper than those two resorts.
hotel st.george is where we stayed

Hi egor110,

Of the Northern Bulgarian resorts, I'd probably suggest looking at Albena as most likely to meet your requirements - couple of photos taken there last July HERE and HERE.

As Fiona has suggested, have a browse through the links that she has given.

Regards,

Baldur

<Edit> Photo of Sveti Sveti Konstantin i Elena, as Wizard suggested, taken August 2004 HERE