This should get you started.
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblAssetCustom.Serialnumber,
tblAssetCustom.Warrantydate,
tblAssets.Userdomain,
tblAssets.Username,
tblADusers.OU
From tblAssets
Inner Join tblAssetCustom On tblAssetCustom.AssetID = tblAssets.AssetID
Left Join tblADusers On tblADusers.Userdomain = tblAssets.Userdomain
And tblADusers.Username = tblAssets.Username
Where tblAssetCustom.State = 1 And tblAssets.Assettype = -1
Order By tblAssets.AssetName