for a start, heres how the spacecraft propulsion systems pan out:
OMS engines are meant to be weak yes, they provide almost 1m/s^2 acceleration when the craft is lightened by losing most of its fuel for ascent
RAMCASTER engines are your primary ascent engines, you spend most of your time with these firing
main engines are quite complex, they have 3 stages:
Normal ext mode. here, they function as normal turbofan (jet) engines, like you would find on an airliner
afterburner ext mode the afterburners ignite, so you have a less fuel-efficient jet engine, like you find on jet fighters
INT mode here, only the afterburners are used (almost at least) and instead of using the atmosphere for air, it relies on internal oxygen stores (because these are used when there is no atmosphere)
now, in your cockpit you actually have an instrument to gauge your ascent profile, and match it to an ideal ascent. look at the central console, to the left of your engine management "MFD" there is a graph. two lines will move across this, one horizontal one that moves up as you ascend, and one vertical one that moves right as you speed up. the intersection of these lines (the +) marks the part of the ascent you are up to.
on the graph, there is a lower zone, a red area, where RAMCASTERS will not function. above this are two curved lines punctuated with circles. between these lines, the ramcasters work with varying levels of efficiency. you should activate RAMCASTER:LO as soon as you enter the first circle (T=1), and then do your best to fly nice and smooth between the two curves of maximum and minimum flight profiles. at the second circle (T=2) switch the RAMCASTERS to HI mode, and follow the same rules
once you reach the T=3 circle, engage the final mode of your main engines and hand over to main engines, relieving the exhausted RAMCASTERS (note, the RAMCASTERS flame out if you fly out of the curved lines, or beyond T=3)
from here, heep the nose below 20* and keep picking up speed, once your ApA reaches 220Km (or something else, mission-appropriate), its MECO time. this should leave you with low enough eccentricity to circularize with OMS engines, just make sure to burn at least 15* before ApA (1m/s^2 isnt much)
from here, just fly as normal, but beware the low thrust. just assume a constant acceleration of 0.9m/s^2 and calculate your burns manually (divide your dV by 0.9 to get the burn time) and get the bird to the station!
if you need anything else, dont hesitate to ask dude!