Dawn of War Codex

I'm a new player so i appreciate anyhelp, my struggle against tyranids is that they field so many units by the time i get a devastator or assault marines i am screwed.

In 1v1 Tyranids are normally going for a heavy T1 and so should you as they don't have a "real" vehicle to threaten you with by fast teching to t2.
Generally you should go with (starting scout), tacticals, scouts as your base. After that it depends on the map and hero matchups.
Again, you most likely want to get a heavy T1 out yourself.

i have a replay with some potentially useful information: https://www.dawnofwar.info/esl/?page=matches&id=346

first off tyranids are one of the hardest matchups for sm, so don't get too discouraged

generally speaking i would get out a dev/asm BEFORE any upgrades like shotguns/wargear

need more specifics for specific help: what sm hero are you playing? what nid hero are you going up against? what build order is the nid using? all of these things have an impact

Another thing imo is to try to focus really heavily on map control instead of trying to win engagements. So instead of throwing down 1 or 2 gens early on and getting upgrades, use your resources to buy more units and bring army sizes to a more balanced state. use your scout mobility to spread out and apply pressure in as many places as you can, they're really key in this matchup. Don't let your tacs/devs get isolated and forced off, because you need to hold map. Try not to let the nid player force you into engagements early on, and use your scout mobility to bait him into your kill zones with debs and tacs behind green cover. It gets easier in t2, when you can get dreads and artillery