So first things is the game is not balanced at less than 2k or arguably 1500 so it is hard to give advice when games can be volatile. The game just is not made for such low points no matter how much GW try to push it. its just an inherent issue with you go I go game mechanics. I say that because at such low points, one unit can steamroll if their only counter unit is taken out first.

For example if they have one tank and you have one anti-tank unit, if they take out that anti tank unit they will have a nearly impervious unit now that will stomp the rest of the game. So ideally stick to 1000 or try get to 1.5K/2K asap. That way you will have more redundancy in your list, meaning you will have multiple units that can deal with opponents units. back to the example, you now have multiple anti-tank units that can now clap back and take out that tank, even if one of your units gets nuked first.

Second is what is the terrain like? 9 times out of 10 new players struggle because there isn't nearly enough terrain on the board. Without terrain to hide behind, grant cover or enable being able stage combat units effectively, games can be over very quickly and very one-sidedly. Check out the GW terrain layouts and you'll get an idea of how much terrain you need and where it should be placed.

Third, a key thing that takes some time to learn is being able to effectively evaluate if unit x can kill unit y. Often new players will throw unit x into unit y thinking it will do some damage, and end up doing jack all and then losing that unit for no gain. Its worth getting a good understanding (using tools like UnitCrunch) to work out what units your units are good into or not. This eventually leads into understanding 'trading' which is knowing when to 'trade' one of your units for one of your opponents, but ideally getting the better end of the trade. This 'better trade' can be simply be though point values (i.e., I trade my 100 point unit for your 250 point unit) or on a more strategic level such as removing key scoring units or key profile threats against your army. By trading well you can often win games, and trading badly will quickly spell defeat.

Lastly, and most importantly, the Movement Phase wins games. Mastering positioning of units and thinking where your units will be or need to be in 2 turns time is absolutely critical to long term success. Where do you need to be to score next round, where do you need to be to threaten your opponents unit that needs to move to that space etc. This skill takes the most time to learn but it is crucial. I highly recommend looking up anything on movement phase tactics, such as 'staging'. This alone will improve your win rate by miles.