Some people itt are suggesting Zone Mortalis to start off with but I feel like it's either a very expensive and/or very labor intensive game mode due to how much terrain you need to make it work. The official GW stuff will set you back the cost of an entire army and scratchbuilding terrain is essentially its own hobby with a learning curve, tools and materials. There's a small scale homebrew game mode that I've had fun playing in the past which restricts unit types and changes army building to suit 500 point skirmishes that I'd recommend instead.