You need to watch a lot of videos and make sure you completely understand what you are doing. It is very simple, but if you don't get all the pieces straight in your head you could end up with hours of frustration.
The "H" screw adjusts the high end of the derailleur .... the outward swing. Loosening it (screwing it out) lets the derailleur swing farther out. Go too far, and the chain falls off the chain ring. Not far enough, the chain rubs, or won't even shift to the large ring from the small.
The "L" screw controls the low range, how far the front derailleur swings inward. Again, too much and the chain drops off on the inside, too little and the chain rubs while on the largest cogs.
Up Front: You want to start with your derailleur cable loose, the barrel adjusters about 1/4 out---some bikes don't have a front barrel adjuster, I guess--and the front derailleur in low with the chain in the middle of the cassette in back.. Pull the cable until it is just snug and tighten it down. Then use the L screw to pretty much center the derailleur and chain over the small chain ring. Hit the shift lever which should tighten the cable and swing the derailleur up and out. Try to center the chain over the large ring with the H limit screw. You might need to come a small amount in or out with either screw --just make Small changes to the limit screws. There is also a "Trim" adjustment on some shift levers, which fine-tunes the derailleur to compensate for the chain angle at the far ends of the cassette. I will find the directions I have, give me a minute ....
Ugh ... my notes are "Low trim ---- Small ring Big cog adjust for clearance. High trim---Big ring big cog, clear, adjust with cable."
So .... on some brifters there are four distinct lever positions, low trim, low, high trim, and high. The trim positions are generally much lighter shifts---sort of a half click.
I assume my notes mean that to set the low trim, you shift all the way down---hit the front lever five or six times just to be sure. Put the chain on the small ring and biggest cog (lowest gear) and set the limit screw ("L" screw") so that the inner side of the front cage is about a millimeter or a little less from the chain.
To set the high trim, hit the lever upwards (On a Shimano brifter, the Up lever) five or six times to be absolutely sure you are all the way in high--then shift Down that half click. The derailleur should move minutely but not drop the chain onto the small cog. Shift to the biggest cog in back, and use the barrel adjuster to loosen (screw In (in Very small amounts---1/16th of turn at a time,) or tighten (screw Out in very small amounts) until t again there is about a millimeter of clearance between the inner cage wall and the chain.
If all that is wrong, sorry. I already told you to watch videos- (don't Ever listen to me.) Park Tools might be the best, but get a few different takes on it.
Again, don't Ever listen to me.