Tomatoes in the ground need to be about three feet apart. Tomatoes are not bushy enough to keep weeds under control, so your tomatoes are overcome with weeds in a month, unless you are constantly hoeing. Hoeing damages the tomato roots, so is not the ideal solution.
The best solution has three parts.
Cover your tomato-bed with, landscape paper sheets of newspaper or weed control fabric. Cut holes or leave gaps about 18 inches across, where the tomatoes are to be planted. If you have already planted them, then sheets of newspaper between them is your best option.
Plant your tomatoes in the holes. Cover the whole bed, including the weed control fabric, newspapers or landscape paper, within 2 inches of the tomatoes with mulch.
You can use mulching you have made yourself or you can buy mulch. Landscape contractors will usually give you a truckload for next to nothing. Buying mulch in bags from a garden center is a very expensive way to mulch your tomatoes.
If you make your own mulch from hedge cuttings, and tree branches it needs to season. Use mulching you made last year, rather than any fresh mulch. This is because fresh mulch absorbs nitrogen from the soil as it begins to break down. If the mulch is absorbing nitrogen your tomatoes will get less.
If you only have fresh mulching and remember any a landscape contractor gives you will be fresh, you need to add a nitrogen-rich fertilizer such as chicken manure pellets and then put the mulch down on top of that. You only need to do this around your tomato plants, where the soil and mulch will be in contact.
Covering the landscape paper with mulching makes it much more attractive. This paper or newspaper will rot down over the summer in any case. If you use weed control fabric you will be able to pull it out from underneath the mulch when your tomatoes are finished and use it again.
As an alternative to woodchip or bark mulch you can use leaf mold or even garden compost, anything to keep the light away from any newly germinated weed seeds and to cover the unsightly paper or fabric covering between the tomato plants. If you use leaf molf from last fall be sure to add chicken manure pellets around the tomato plants before you put the leaf mold down.
Unless you KNOW that your own compost is totally weed free it is best to avoid using that as a weed control mulch. If you have had problems in the past with weeds growing in your compost, then using it to mulch tomatoes will make the weed problem worse instead of better.
The lowest cost materials to use underneath the compost or mulch are newspaper or black polythene sheeting, the best are landscape paper or weed control fabeics.