Try to use your harass to also clear waves. Just do your best to keep minions between you and your opponent early game. The major exception to that is if it'd make you go near the edge of the lane. It's usually best to stick around the middle area or towards sides you have warded due to low mobility. Just be careful if you start trying to shove the lane without vision. You can easily get killed by a jungler gank.
Other than that, just use Q to poke at enemy and kill minions. Use autos when you can't Q to kill a group of minions(or a minion+harass), low on mana, or want to to gain mana from passive. I mainly don't use W in lane unless I land a stun on enemy champ(which I drop W then Q while they're stuck), or to finish a group of low minions while Q is down. Also don't be afraid to let Q fully charge then let it cancel. Better to get the refunded mana than blast at nothing unless you need to quickly stop charging to run.
Also if you're not pushed into your tower, take time to look to top or bottom lanes for pressure. Your ult can provide some great help in getting them a kill or scaring off enemy to save your team.
You're Xerath, try to use that range to its fullest. Sit back and poke away where you're safe. Eventually, you'll get the items to really deal some damage. Just play safe, help others if you feel you can, and keep your flanks warded until that point if you can't get enough constant poke down on your opponent laner.